v0.5.0

• use /scratch rather than /tmp so buildkitd can use the overlayfs snapshotter
  • significant performance boost; observed one build go from 8 minutes to 4 minutes
  • thanks to @jmccann for noticing this! (#50)
• support for $REGISTRY_MIRRORS (comma-separated list)
• pass --debug to buildkitd if debug is set
• bump buildkit to v0.8.0
• bump rootlesskit to v0.11.1
• bump go-containerregistry to v0.3.0