Merge pull request docker#155 from tiborvass/vendor-buildkit

vendor: update buildkit to docker-19.03 (ae10b292)

Merge pull request docker#124 from tiborvass/update-docs

Update docs

Merge pull request docker#87 from tiborvass/no-build-field

[Carry docker#79] Change compose file handling to require valid service specifications

Merge pull request docker#83 from tonistiigi/relative-dockerfile-path

bake: make dockerfile relative to context

Merge pull request docker#51 from tonistiigi/travis

add releases to travis