v0.6.0

• @tomjw64 added support for labels (#42) - thanks!
  • configure as LABEL_* and LABELS_FILE, same as build args
• support building additional targets, i.e. to run tests (fixes #52)
  • configure as ADDITIONAL_TARGETS: target1,target2
  • if outputs: are also configured with names matching the targets, the target's image will be saved to the output in same way as the image output