Add support for setting procMount of build container

What does this MR do?

This MR adds support for setting the procMount value of the build container's security context.

Why was this MR needed?

This is needed to:

1. Permit rootless containers, and
2. Run Nix without privileged: true.

See also:

• https://blog.jessfraz.com/post/building-container-images-securely-on-kubernetes/
• https://github.com/opencontainers/runc/issues/1658