Use Crane for container image building

[SikaGrr]

Description

Build container images using Crane.
This is behind a feature flag CRANE_WORKLOADS_ENABLED.
Handling of .dockerignore planned in a followup: b/481653975

Issue

b/481654085

Testing

• Unit tests (more tests to cover .tar archive building planned in b/481653975)
• Goldens
• Manual testing:

# from script directory:
xpk workload create ...  --workload test --command "./script.sh"
# script contents:
echo "hello from script"
ls -al

Script output (w/ Crane): screen/6jefyNtPBA5Qh63
Script output (w/ Docker): screen/BsNCJkDkjk9w3DC
File owners match (root), as well as WORKDIR.