Prepare devcontainers repository for public

[yevhenkuznetsov]

In order to pull devcontainers without permissions, the following action items has to be fulfiled:

• Document and add examples how images can be used to start a development container

As a nice to have feature:

• Add an additional jobs for PRs to build images before merging. Re-use images after PR merge to speed-up the deployment.

Depends on #10