Support ARM64 based CPU

Describe the bug

As there isn't a build for ARM64 based CPUs, Postal cannot run on them and errors with "exec user process caused: exec format error".

To Reproduce

1. Use an ARM64-based server (Raspberry Pi, Hetzner Cloud ARM server, etc.)
2. Attempt to create a container with the Postal docker image
3. Observe error

Expected behaviour

I expect Postal to work on ARM. While the ARM architecture wasn't used as much in servers previously, it's becoming increasingly popular with many dedicated/cloud server providers offering them.

Additional information/context

I've created a PR to add ARM builds in #2770.