Use github actions to build docker image

This is an alternative to #112 (closed)

An example has been implemented in a fork: https://github.com/MatthewJohn/terrareg/commit/669efe6a207458c74e20f702424414e0e17c9302

I believe we would need to create a NOTICE file (see https://infra.apache.org/licensing-howto.html), outlining the licenses of the installed binaries in the docker image (e.g. terraform-docs etc.).

Should also note that all licenses for included software can be found in /usr/share/doc/*/copyright of the docker images and note to the base image used.