feat: add git:from-image support

The new command superscedes the previous tags plugin, and integrates docker image deployment with the general build process.

While `docker image load` is not supported, this otherwise completely handles all previous workflows supported by the `tags:deploy` command, while doing so in a much easier to use interface.

Closes #4296

plugins/git/help-functions

@@ -29,6 +29,7 @@ fn-help-content() {
   cat <<help_content
     git:allow-host <host>, Adds a host to known_hosts
     git:auth <host> [<username> <password>], Configures netrc authentication for a given git server
+    git:from-image <app> <docker-image> [<git-username> <git-email>], Updates a app's git repository with a given docker image
     git:sync [--build] <app> <repository> [<git-ref>], Clone or fetch an app from remote git repo
     git:initialize <app>, Initialize a git repository for an app
     git:public-key, Outputs the dokku public deploy key