Add checksum feature

[kindermax]

This PR is intended to add a feature called checksum (may be changed).

It is a WIP (test will be written after we make sure the implementation is correct)

This is how it works.

1. add to checksum list of files

webapp:
    image: webapp-dev
    build:
      dockerfile: ./docker/Dockerfile.webapp-dev
      checksum:
      - ./package.json
      - ./package-lock.json
      - ./docker/Dockerfile.webapp-dev
      context: .
    working_dir: /work

2. If any of those files changed, rebuild the image

For now, I create file in ~/.docker/checksums.json. This place is chosen because I did not found any better place. If you know where is the better place - I will be glad to hear.

I am calculating the checksum for image and write it to checksums.json. Then I recalculate checksum on every run, and if something changed, update checksum and rebuild the image.

UPD. also i've added new command called checksum which calculates and prints same checksum as it will be used to make the decision to rebuild an image

usage:
docker-compose checksum my_service

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "add-checksum-feature" [email protected]:kindritskyiMax/compose.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354237720
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[kindermax]

#6936 my issue