Fix action metadata

1. Update README.md ( #14 )
2. Fix action metadata ( #12 )

Add output parameters

1. Add target option for multi-stage builds ( #8 )
2. Export computed tag for a further action ( #9 )

Fix bugs and improve tags resolving

1. Fix option name for cache ( #5 )
2. Use registry input in image tags ( #2 )

Support build-time variables

1. Add support for docker build-time variables (--build-arg)
2. Use fixed docker image tag instead latest

First working stable version

This action helps build, tag and publish your docker image. The main feature differs from other actions is to use semantic version tag to generate docker image tags.