New workflow for image build

[trevorcampbell]

We are likely going to merge UBC-DSCI/introduction-to-datascience-python#241 in the python version, which will build docker images on PRs to Dockerfile (also need to add main branch protection to avoid accidentally circumventing the workflow) instead of on commits to Dockerfile. That way we can easily review PRs with a stable image without needing to modify images interactively (and have a new image built when the PR merges, which might differ from the test environment)

If that works well there, we should eventually port it over to here too.