AWS SAM running local won't resolve FindInMap Intrinsic

### Description:

I can't make a template.yaml with Mappings work when testing locally, although they will deploy and work as expected.

### Steps to reproduce:

1. `sam init --runtime python3.8`
2. Insert Mappings such as the following in `template.yaml`

```yaml
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
sam-app

Sample SAM Template for sam-app

Parameters:
Environment:
Description: 'Required. Environment'
Type: String
AllowedValues:
- test
- prod
Default: test

# More info about Globals: https://github.com/awslabs/serverless-application-model/blob/master/docs/globals.rst
Globals:
Function:
Timeout: 3
MemorySize: !FindInMap [Environments, !Ref Environment, MemorySize]

Mappings:
Environments:
test:
MemorySize: 512
prod:
MemorySize: 2048

Resources:
HelloWorldFunction:
Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
Properties:
CodeUri: hello_world/
Handler: app.lambda_handler
Runtime: python3.8
Events:
HelloWorld:
Type: Api # More info about API Event Source: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api
Properties:
Path: /hello
Method: get

Outputs:
# ServerlessRestApi is an implicit API created out of Events key under Serverless::Function
# Find out more about other implicit resources you can reference within SAM
# https://github.com/awslabs/serverless-application-model/blob/master/docs/internals/generated_resources.rst#api
HelloWorldApi:
Description: "API Gateway endpoint URL for Prod stage for Hello World function"
Value: !Sub "https://${ServerlessRestApi}.execute-api.${AWS::Region}.amazonaws.com/Prod/hello/"
HelloWorldFunction:
Description: "Hello World Lambda Function ARN"
Value: !GetAtt HelloWorldFunction.Arn
HelloWorldFunctionIamRole:
Description: "Implicit IAM Role created for Hello World function"
Value: !GetAtt HelloWorldFunctionRole.Arn
```

3. `sam build --parameter-overrides ParamaterKey=Environment,ParameterValue=test --use-container`
4. `sam local start-api --debug`, and it will show unresolved references.

```
2021-03-06 00:30:42,394 | local start-api command is called
2021-03-06 00:30:42,399 | Collected default values for parameters: {'Environment': 'test'}
2021-03-06 00:30:42,417 | Unable to resolve property MemorySize: OrderedDict([('Fn::FindInMap', ['Environments', OrderedDict([('Ref', 'Environment')]), 'MemorySize'])]). Leaving as is.
2021-03-06 00:30:42,417 | 2 resources found in the stack
2021-03-06 00:30:42,417 | Collected default values for parameters: {'Environment': 'test'}
2021-03-06 00:30:42,433 | Unable to resolve property MemorySize: OrderedDict([('Fn::FindInMap', ['Environments', OrderedDict([('Ref', 'Environment')]), 'MemorySize'])]). Leaving as is.
2021-03-06 00:30:42,434 | Collected default values for parameters: {'Environment': 'test'}
2021-03-06 00:30:42,449 | Unable to resolve property MemorySize: OrderedDict([('Fn::FindInMap', ['Environments', OrderedDict([('Ref', 'Environment')]), 'MemorySize'])]). Leaving as is.
2021-03-06 00:30:42,449 | Found Serverless function with name='HelloWorldFunction' and CodeUri='HelloWorldFunction'
2021-03-06 00:30:42,452 | Collected default values for parameters: {'Environment': 'test'}
2021-03-06 00:30:42,467 | Unable to resolve property MemorySize: OrderedDict([('Fn::FindInMap', ['Environments', OrderedDict([('Ref', 'Environment')]), 'MemorySize'])]). Leaving as is.
2021-03-06 00:30:42,467 | 2 resources found in the template
2021-03-06 00:30:42,468 | Found '1' API Events in Serverless function with name 'HelloWorldFunction'
2021-03-06 00:30:42,468 | Detected Inline Swagger definition
2021-03-06 00:30:42,468 | Lambda function integration not found in Swagger document at path='/hello' method='get'
2021-03-06 00:30:42,468 | Found '0' APIs in resource 'ServerlessRestApi'
2021-03-06 00:30:42,468 | Removed duplicates from '0' Explicit APIs and '1' Implicit APIs to produce '1' APIs
2021-03-06 00:30:42,468 | 1 APIs found in the template
2021-03-06 00:30:42,471 | Mounting HelloWorldFunction at http://127.0.0.1:3000/hello [GET]
2021-03-06 00:30:42,471 | You can now browse to the above endpoints to invoke your functions. You do not need to restart/reload SAM CLI while working on your functions, changes will be reflected instantly/automatically. You only need to restart SAM CLI if you update your AWS SAM template
2021-03-06 00:30:42,471 | Localhost server is starting up. Multi-threading = True
2021-03-06 00:30:42 * Running on http://127.0.0.1:3000/ (Press CTRL+C to quit)
```

### Observed result:

5. Insist with `curl http://127.0.0.1/hello` and sam will show an error:

```
2021-03-06 00:33:25,099 | Found one Lambda function with name 'HelloWorldFunction'
2021-03-06 00:33:25,099 | Invoking app.lambda_handler (python3.8)
2021-03-06 00:33:25,099 | No environment variables found for function 'HelloWorldFunction'
2021-03-06 00:33:25,099 | Environment variables overrides data is standard format
2021-03-06 00:33:25,099 | Loading AWS credentials from session with profile 'None'
2021-03-06 00:33:25,113 | Resolving code path. Cwd=/home/ricardo/Sandbox/learning/aws/sam-app/.aws-sam/build, CodeUri=/home/ricardo/Sandbox/learning/aws/sam-app/.aws-sam/build/HelloWorldFunction
2021-03-06 00:33:25,113 | Resolved absolute path to code is /home/ricardo/Sandbox/learning/aws/sam-app/.aws-sam/build/HelloWorldFunction
2021-03-06 00:33:25,113 | Code /home/ricardo/Sandbox/learning/aws/sam-app/.aws-sam/build/HelloWorldFunction is not a zip/jar file
2021-03-06 00:33:25,127 | Skip pulling image and use local one: amazon/aws-sam-cli-emulation-image-python3.8:rapid-1.19.1.

2021-03-06 00:33:25,127 | Mounting /home/ricardo/Sandbox/learning/aws/sam-app/.aws-sam/build/HelloWorldFunction as /var/task:ro,delegated inside runtime container
2021-03-06 00:33:25,127 | Cleaning all decompressed code dirs
2021-03-06 00:33:25,128 | Exception on /hello [GET]
Traceback (most recent call last):
File "/home/ricardo/.local/lib/python3.8/site-packages/docker/utils/utils.py", line 415, in parse_bytes
digits = int(digits_part)
ValueError: invalid literal for int() with base 10: "OrderedDict([('Fn::FindInMap', ['Environments', OrderedDict([('Ref', 'Environment')]), 'MemorySize'])])"

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/ricardo/.local/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
response = self.full_dispatch_request()
File "/home/ricardo/.local/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/home/ricardo/.local/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/home/ricardo/.local/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
raise value
File "/home/ricardo/.local/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
rv = self.dispatch_request()
File "/home/ricardo/.local/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/home/ricardo/.local/lib/python3.8/site-packages/samcli/local/apigw/local_apigw_service.py", line 297, in _request_handler
self.lambda_runner.invoke(route.function_name, event, stdout=stdout_stream_writer, stderr=self.stderr)
File "/home/ricardo/.local/lib/python3.8/site-packages/samcli/commands/local/lib/local_lambda.py", line 111, in invoke
self.local_runtime.invoke(config, event, debug_context=self.debug_context, stdout=stdout, stderr=stderr)
File "/home/ricardo/.local/lib/python3.8/site-packages/samcli/lib/telemetry/metric.py", line 216, in wrapped_func
return_value = func(*args, **kwargs)
File "/home/ricardo/.local/lib/python3.8/site-packages/samcli/local/lambdafn/runtime.py", line 148, in invoke
container = self.create(function_config, debug_context)
File "/home/ricardo/.local/lib/python3.8/site-packages/samcli/local/lambdafn/runtime.py", line 82, in create
self._container_manager.create(container)
File "/home/ricardo/.local/lib/python3.8/site-packages/samcli/local/docker/manager.py", line 95, in create
container.create()
File "/home/ricardo/.local/lib/python3.8/site-packages/samcli/local/docker/container.py", line 167, in create
real_container = self.docker_client.containers.create(self._image, **kwargs)
File "/home/ricardo/.local/lib/python3.8/site-packages/docker/models/containers.py", line 860, in create
create_kwargs = _create_container_args(kwargs)
File "/home/ricardo/.local/lib/python3.8/site-packages/docker/models/containers.py", line 1077, in _create_container_args
create_kwargs['host_config'] = HostConfig(**host_config_kwargs)
File "/home/ricardo/.local/lib/python3.8/site-packages/docker/types/containers.py", line 182, in __init__
self['Memory'] = parse_bytes(mem_limit)
File "/home/ricardo/.local/lib/python3.8/site-packages/docker/utils/utils.py", line 417, in parse_bytes
raise errors.DockerException(
docker.errors.DockerException: Failed converting the string value for memory (OrderedDict([('Fn::FindInMap', ['Environments', OrderedDict([('Ref', 'Environment')]), 'MemorySize'])])) to an integer.
```

### Expected result:

There shouldn't be unresolved FindInMap causing exceptions, just as it doesn't happen with CloudFormation itself.

### Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS: Pop_OS! version 20.10, based on Ubuntu.
2. `sam --version`: 1.19.1 and 1.20.0
3. AWS region: not sure if relevant, but I deploy to us-west-2.

Start with the reproduced template.yaml and run sam build followed by sam local start-api --debug using the reported FindInMap and Ref values. Trace the local template resolution path shown by the SAM CLI stack trace, then verify that curl no longer passes an unresolved FindInMap value to Docker and the function starts successfully.

Written by the indexing model from the issue text.