gracefully shut down builder process

[syphar]

Right now when you would just restart the builder, we would just

• stop tracking the background docker run call
• after restart, pick up the build again from the queue (or another builder does)
• and start from scratch.

This means some time after a "simple" restart, we still have the old build running and consuming resources (cpu, memory, disk).

The current workaround is

1. lock the queue
2. wait until all in-progress builds are finished
3. restart the server processes
4. unlock the queue

What woud actually be the best option is:

• extend https://github.com/rust-lang/rustwide so it supports cancelling an in-progress build (probably just SIGTERM or SIGKILL the docker run process.
• change the builder so it can listen to the signals,
• update the in-progress records in the builds table
• kill the in-progress builds on shutdown

See also #3250