ENP-2825: Automatically trigger builds on push

[lachlancooper]

Summary

Automatically trigger builds of all images via actions workflows on push to any branch.

If the pushed branch is the default, the images will be released using their current version tag.

This also bumps the version of all external actions to the latest.

Problem description

Currently, image builds are triggered manually and individually. This has multiple issues:

• A branch can be broken with no feedback at PR review time.
• Testing is manual and it's easy to miss one or more builds.
• Releasing is manual and it's easy to miss one or more builds after merge.

Pros/cons of approach implemented

• Removes the ability to trigger individual builds/releases manually.
  • The overall workflow can still be triggered manually if needed.
• The base protoc image typically needs to be built and released before the individual images (e.g. protoc-cpp).
  • This workflow restricts the build to one job at a time to ensure protoc is built first.
  • This could be improved with reusable workflows, i.e. build protoc in a separate workflow to the other images.

Checklist

• Is this PR a reasonable size?

• List deployment sequence with all relevant PRs.
  • ...

---

Code Review Guidelines for Reviewers

• Try to review in a timely manner. Opinions/nitpicks should not be blockers. Pair on a call for non-trivial feedback.
• Overall design and approach should follow established patterns. Don't try to make the PR perfect.
• Try to identify edge cases, race conditions, over-engineering, lack of test coverage and complexity.
• If you don't feel qualified to review the code, pass it on to someone who is.