[GRASS-git] [OSGeo/grass] 68f2c8: Add github workflow for docker build and push (#1790)

[mmacata]

  Branch: refs/heads/releasebranch_7_8
  Home:   https://github.com/OSGeo/grass
  Commit: 68f2c87a02e1b9bfb29a3cfc7a1951e21851d07b
      https://github.com/OSGeo/grass/commit/68f2c87a02e1b9bfb29a3cfc7a1951e21851d07b
  Author: mmacata <mmacata at users.noreply.github.com>
  Date:   2021-08-19 (Thu, 19 Aug 2021)

  Changed paths:
    A .github/workflows/docker.yml
    A docker/alpine/Dockerfile
    R docker/alpine/Dockerfile_alpine
    M docker/alpine/README.md
    A docker/debian/Dockerfile
    R docker/debian/Dockerfile_debian_pdal
    M docker/debian/README.md
    A docker/ubuntu/Dockerfile
    R docker/ubuntu/Dockerfile_ubuntu_pdal
    M docker/ubuntu/README.md

  Log Message:
  -----------
  Add github workflow for docker build and push (#1790)

* init gha for docker

* update tag name

* remove test branch/repository

* enhance information