Test recc with buildbox-run-userchroot

Description

This adds general support for testing recc with sandboxed runner environments based on an Alpine Docker image. It furthermore adds userchroot support and enables a test run of recc with buildbox-run-userchroot.

Validation

The log of a successful test job with this branch can be viewed at: https://gitlab.com/BuildGrid/buildbox/buildbox-common/-/jobs/1557726259

Issues addressed

Part of #11 (closed)