Set up CI

[nya3jp]

Currently platform contributors verify Docker builds locally. This is probably okay before the contest, but I'm afraid that it's undesirable during the contest because any change to Dockerfiles may break other team's solutions.

It is great if we could set up CI to verify Dockerfiles on pull requests. It would do something like:

• For each platform:
  • Build a base image with Dockerfile.base if it exists
  • Build one or more test solutions using Dockerfile and the base image, without network access

[beevee]

As mentioned in #25, we have an internal CI that continuosly checks all starterkit repos against current Dockerfiles. But not PRs, you're right. I will consider setting up CI, thank you!