Exceptions for files and directories - File Sync

I have ruby project and use sync tool.

The directories in project are:
- app
- config
- db
- lib
- spec
and etc

Dockerfile (simplified)
```
FROM ruby

RUN mkdir -p /backend

WORKDIR /backend

COPY Gemfile Gemfile.lock ./
RUN bundle install

COPY . .

RUN bundle exec assets:precompile
```

skaffold.yaml
```yaml
apiVersion: skaffold/v2beta1
kind: Config

build:
artifacts:
- image: backend-dev

profiles:
- name: dev
activation:
- command: dev
build:
artifacts:
- image: backend-dev
sync:
manual:
- src: 'app/**/*'
dest: '/backend'
- src: 'config/**/*'
dest: '/backend'
- src: 'db/**/*'
dest: '/backend'
- src: 'lib/**/*'
dest: '/backend'
- src: 'spec/**/*'
dest: '/backend'
- src: 'README.md'
dest: '/backend'
- src: 'run.sh'
dest: '/backend'
```
And here are not all the directory and files that do not need to rebuild.

I need to rebuild when changing only Dockerfile, Gemfile, Gemfile.lock.
It would be great to have exception, for example
```yaml
sync:
except:
- Dockerfile
- Gemfile
- Gemfile.lock
```
Thus, the container was build only when these files modified. The rest of the files would just sync.
May be it's too much and possible some regex can be done for this? Or something else?
Thanks for the cool project!

- Skaffold version: v1.7.0
- Operating system: Ubuntu 18.04