fix(orchestration): ignore image prune failure in build unit

A rebuild leaves the previous image dangling while the old container still uses it, so the ExecStopPost podman image prune -f exits 125 and systemd marks juice--build.service as failed even though the build succeeded. Prefix the command with - so systemd ignores its exit status; the dangling image is cleaned up on a later rebuild.

Closes #399 (closed)

Closes #399 (closed)