Add Docker file validation on PRs

[dabutvin]

We should use github actions for this. We are building and running tests on appveyor right now on PRs, but this does not include the docker file.

I think eventually we should move everything to github actions, but this would be a good start and give us extra coverage.

I just want to make sure whatever changes are made still let the compression container build so something like this should work

$ docker build -f Dockerfile.CompressImages . -t imgbot

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dabutvin]

bump

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dabutvin]

Bump