Dokku Apt Versions Save

• fix: always bust cache when using a dpkg-packages file

• docs: update requirements and add note about compatible extended app images
• feat: implement package cache using labels

fix: labels in --change flags must have a value, even if empty

• feat: properly label intermediate containers for later cleanup
• chore: run shfmt

• feat: add apt-preferences support
• docs: use code in headers

• feat: add support for apt-env

• shfmt
• add support for apt-sources-list #29

handle DOKKU_TRACE