Bug: Missing DockerContext doesn't fail validation

### Description:

If you set `Dockerfile` in a function's metadata, but _not_ `DockerContext`, `sam build` seems to just work, except then `sam local invoke` complains `Error: ImageUri not provided for Function: YourFunctionName of PackageType: Image` which is very confusing. Note also `sam validate` doesn't complain abou this.

### Steps to reproduce:

Create a correct docker image SAM template, then remove just the `DockerContext` metadata param.

### Observed result:
`sam local invoke` fails with a confusing error

### Expected result:
`sam validate` fails, or maybe `sam build`?

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

1. OS: MacOS
2. `sam --version`: 1.116.0

```
{
"version": "1.116.0",
"system": {
"python": "3.11.6",
"os": "macOS-14.0-arm64-arm-64bit"
},
"additional_dependencies": {
"docker_engine": "23.0.6",
"aws_cdk": "Not available",
"terraform": "1.4.1"
},
"available_beta_feature_env_vars": [
"SAM_CLI_BETA_FEATURES",
"SAM_CLI_BETA_BUILD_PERFORMANCE",
"SAM_CLI_BETA_TERRAFORM_SUPPORT",
"SAM_CLI_BETA_RUST_CARGO_LAMBDA"
]
}
```