Docker images for CI outlined in internal docs and contributor guide

[nuke-web3]

@NukeManDan @imadarai I finally get the credential to use paritytechnologies organization to push the docker image. After pushing, it will also be shown at: https://hub.docker.com/u/paritytechno

Note that I have updated the credentials of docker login in github environment secrets, so the CI in main and all other branches pushing to jimmychu0807/substrate-docs will fail.

Originally posted by @jimmychu0807 in https://github.com/substrate-developer-hub/substrate-docs/pull/397#discussion_r734213613

[nuke-web3]

Who maintains this image? #1051 is related - @jimmychu0807 @TriplEight any idea?

We do not use this at all presently, all wrokflows can be deleted at this time for #674 to replace.

[TriplEight]

Please see details in #1051, we don't have creds from this dockerhub registry.