Use the GitHub release data to decide if an upgrade is available.

Only download the release if a newer stable release is published.

Install the full release - including the DNS scripts (requires "make")

Don't check for upgrades when restarting.

Remove --keep logic - only 1 old version is kept, only if make isn't available.
Old versions are availablel from the repo; if you have local changes, --upgrade doesn't make sense.

Can upgrade (or downgrade) to any tagged release with --experimental vx.yy

Note that GitHub API requests are rate-limited; this shouldn't be an issue
unless getssl is run more than ~60 times/hr.  If the limit is exceeded,
getssl will sleep until the limit is reset.  The limit is per-IP address.

The tests have been modified to only check for updates in the tests that
verify the check for updates logic.  This has the side effect of not
doing MANY pointless checks for updates in the tests...

Some versions of nginx create directories, so use -p with mkdir.

Note: To add the secrets to your repository on GitHub

Goto Actions -> Settings -> Secrets

New Repository Secret

Add DUCKDNS_TOKEN
Add DYNU_API_KEY

Reference:
- https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#cmd

> The `CMD` instruction should be used to run the software contained in
your image, along with any arguments. `CMD` should almost always be used
in the form of `CMD ["executable", "param1", "param2"…]`

This change will drop the history of the repository when cloning, which
will save bandwidth and disk space, speed up the clone and checkout
process during the test and Docker build process.