Build Docker image and push to Dockerhub

[guidopetri]

As mentioned in #1051, the CircleCI docker build step isn't working, and we want to move it to a Github Action instead. This PR has two commits:

• first, revert the CircleCI additions so that the CircleCI tests file is clean (via reverting the two commits from update circleci to build image and push to dockerhub, pending approval #1065)
• second, add a build+push to dockerhub github action.

This will require two secrets that I believe can be set in the repository settings somewhere: one for a DOCKERHUB_USERNAME and another for a DOCKERHUB_TOKEN. I also went ahead and added both an amd64 and an arm64 build here, and am hoping that it just works (tm).

I also once again set the build and push to only happen when main is pushed to / gets new commits, but I think we can change that to work only on this branch from my fork if we want to test before merging.

[guidopetri]

@mockdeep hopefully it works this time!

[mockdeep]

Thanks so much for this! I think this will help a lot with environment issues.

[mockdeep]

I've set these secrets, so hopefully that'll work.

[guidopetri]

I believe that this cache-from will cache to/from the Github Actions environment, so there's no need to pull an image locally, but I'm not 100% confident.

[guidopetri]

Updated the PR with your suggestions and rebased them into the second commit. Should I keep the run-tests name on CircleCI instead of reverting to build?

[mockdeep]

Let's give it a try! I'm wondering why we don't see the image building on this PR, but maybe we need to merge it before it will run.

Should I keep the run-tests name on CircleCI instead of reverting to build

I think build is fine for now. We can always revisit later.

[guidopetri]

It worked?!!

[mockdeep]

Oh, I didn't even think to look. It did!

[mockdeep]

https://hub.docker.com/repository/docker/stringerrss/stringer/general