use GitHub actions for docker builds

[ChristianHeimke]

about / background

This PR enables Github action to build all released strapi versions as docker image and push it to the docker hub. It will skip the build process if a tag already exists.
It is more "docker-like" and make the build process more transparent for others.

usage

The workflow will run every night automatically, pull the current strapi releases from Github and grab the current image tags from dockerhub. It will generate a build matrix based on the strapi versions, nodejs versions (basic and alpine) to build and the flavour of the image (base or strapi). The build process will be skipped if the image tag already exists or contains the keyword beta (needs to be skipped because yarn can't find the beta package).

demo images

you can find the testing builds here: https://hub.docker.com/r/loumaris/strapi/tags

todo

If you accept the PR some small updates are needed:

• docker hub path needs to be changed to strapi/strapi
• add docker credentials as secrets for pushing the image
• update github/docker repository to a community driven project to increase the limits
• update/change badges in README.md
• maybe skip / remove older tags

[strapi-cla]

All committers have signed the CLA.

[magisters-cc]

What about node 16? As i know, Strapi v4 works on node v16.

[Eventyret]

Would suggest set node-14 and 16