Adds GH workflow to build docker images on version tags

[vpereira01]

Adds a GitHub workflow, and custom docker file, using docker buildx which builds docker container images for different CPU architectures and publishes these images to GitHub packages.

Workflow will automatically run when a version tag is created, pattern v*, example v0.23.0-beta. Also supports manual workflow trigger to allow the flexibility to build images without needing to create a new version tag.

Fixes: #1623

Example of how packages are listed from my fork https://github.com/vpereira01/btcd/pkgs/container/btcd

Few notes:

• The step, Extract metadata (tags, labels) for Docker, will generate the tag latest when creating a tag v0.99 on master. Example from my fork: https://github.com/vpereira01/btcd/runs/5561633723?check_suite_focus=true . Not sure if this should be kept given the comments on wire: add proper types for flag field and improve docs #1632
• I think the packages must be made public. GitHub seems to default packages to be private so anonymous docker pulls will not be allowed.
• I have not used image digests on Dockerfile given the digest varies according to the TARGETARCH.
• The platforms linux/amd64,linux/arm64,linux/arm,linux/386 seems good enough to cover must of uses cases. To support specific GOARM a lot rework is required.

References:

• https://www.docker.com/blog/docker-v2-github-action-is-now-ga/
• https://docs.github.com/en/actions/publishing-packages/publishing-docker-images#publishing-images-to-github-packages
• Possible optimizations https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/

[jcvernaleo]

Three super minor comments.

[jcvernaleo]

Looks good now.

OK