Use Github hosted ARM runners to build arm64/aarch64 release image (#779)

Use Github hosted ARM runners to build arm64/aarch64 release image (#779)

Why this change?

We have been given access to Github's private beta of ARM hosted
runners. Switching to ARM runners should drastically speed up the time
required for us to build our ARM image.

What does this change do?

1. Switch to use Github's ARM hosted runners.
2. Build release image for arm64 as well. We previously only built the
   slim image because building the release image through emulation is
   way too slow so we skipped the release image.
3. Update `bundle` in `release.Dockerfile` to install gems in parallel
   based on the number of cores instead of hardcoding it to 4 jobs.