Add support for multi-arch Docker images

[webvictim]

Fixes #3384

This PR:

• adds support for building arm and arm64 Docker images using processors with amd64 architecture
  • this is done using the binfmt_misc kernel extension which automatically detects the ELF 'magic' for binaries from other architectures and uses qemu-user-static to emulate the correct CPU and run them
• adds support for multi-arch Docker manifests
  • essentially this means there is one image/tag name which has 3 different images behind it (amd64, arm and arm64)
  • the correct image for the host architecture will be picked by the Docker daemon when the image is pulled
• tidies up buildbox and image Dockerfiles by sorting them more logically\
• removes the separate FIPS Dockerfiles in favour of a multi-stage Docker build which optionally overrides the ENTRYPOINT for FIPS images (we were already using this for teleport-docker-cron)
• removes separate release-arm and release-arm64 steps in favour of automatically deducing the correct arch from the supplied ARCH variable
  • this will cut down on boilerplate and make it a little easier to add support for more architectures in future

Still to come:

• a separate PR for updating the nightly teleport-docker-cron images which are produced by Drone
  • the logic changes are quite complicated and I didn't want to overcomplicate this PR

[benarent]

This PR will be really helpful for a project I'm working on. Commenting with enthusiasm and to watch this PR

[benarent]

Another thing I found out, when we publish the different version Quay provides this handy UI feature.

[webvictim]

I'll try and have another look at this tomorrow.

[russjones]

@webvictim We have not had a lot of progress on this, so I am going to close it for now. Please re-open if you want to work on this issue again.