[CI] Add workflow to push Docker image to Dockerhub and ECR

[ErikSchierboom]

This PR adds a GitHub Actions workflow to automatically push a new version of this repo's Docker image to both Dockerhub and ECR.
The former is used when running the tooling on your local machine using the development environment, the latter is currently used when running the tooling on our staging environment (https://exercism.lol/) but will eventually be used for our production website.

We've sent PRs to tooling repos before related to this functionality, but those PRs added two separate workflows for pushing to Dockerhub and ECR. With this PR, these workflows have been merged into a single workflow, which is much more efficient. If present, the old workflows will be removed in this PR.

See exercism/v3#2969

[ErikSchierboom]

@ynfle It looks like you don't have review permissions for this repo. Would you be interested in having those permissions?

[ee7]

Nice - this PR also upgrades docker/build-push-action to v2.

And it's great that this uses actions/cache rather than satackey/action-docker-layer-caching, which caused some CI errors previously (see https://github.com/exercism/nim-test-runner/runs/1382436417).

Approved, conditional on the docker/build-push-action version number being fixed.

[ee7]

Suggested change

	multiple-registries:
	multiple-registries:

Non-blocking comment: I think this name could be better.

[ynfle]

@ynfle It looks like you don't have review permissions for this repo. Would you be interested in having those permissions?

Which permissions? Didn't I used to have permissions?

[ErikSchierboom]

Hmmm, there is something weird here. Both of you have approved, but they don't count as approving reviews. @iHiD could you check if the permissions are correct for @ee7 and @ynfle?

[ee7]

Hmmm, there is something weird here. Both of you have approved, but they don't count as approving reviews.

Isn't it just because the CODEOWNERS file contains this?

nim-test-runner/.github/CODEOWNERS

Line 1 in aba25f9

* @exercism/maintainers-admin

Maybe a member of that team has to approve even when the PR was created by somebody in that team?

check if the permissions are correct for ee7 and ynfle

From the GitHub API I can confirm that the permissions in this repo are:

Permission	Users
push and pull	ee7, ynfle, ErikSchierboom
admin	iHiD, kytrinyx