scripts: Add scripting support for start_rust_buildenv

[binxing]

start_rust_buildenv currently supports interactive invocation only. This commit adds the following features.

• Allow user to specify command to execute, by appending -- followed by the actual command.
• Container started by custom command will be created with --rm --init appended to docker run - i.e., the container will be removed once exited.
• Custom commands are supported by existing containers too. This is done by passing through the command line to docker exec.

[billionairiam]

I just tried U=. start_rust_buildenv -- ./build and U=. start_rust_buildenv -- ./build -r -Ffull. It works for me.

[binxing]

@billionairiam, I'm taking your feedback as an approval to merge.