Fix for issue #420, entrypoint has a race condition where installation fails if DB is not ready in time.

[dghodgson]

The NextCloud installer seems to set a default DB user and password in config.php if the first installation attempt fails (see issue nextcloud/server#41074 ) . This PR gets around that issue by waiting for the DB service to become available before proceeding with installation.

This adds wait-for-it to /usr/bin in the dockerfile templates. The script is called in entrypoint.sh right before installation is attempted.

A helper function get_host_string() is also added to set default ports for MySQL/Postgres if no port is provided in the DB host string.

I attempted to implement the wait_for() function from wait-for-it into the entrypoint script directly, but ran into various issues (including the use of /bin/sh by entrypoint.sh instead of /bin/bash, which complicated things a bit). Calling the script just works without any issues, but that does mean adding another file to the docker image.

Not sure if running the update script to update the files in 19/20/21 is required as part of the pull request, so I opted not to.

[dghodgson]

Seeing the alpine images failing due to a lack of bash.

Bash provides /dev/tcp which can be used to test network connections without an added dependency like netcat. Will have to look into another solution that works on alpine.

[J0WI]

Closing this because ADD is discouraged, especially from url, and the script doesn't work anyway on Alpine. IMHO it's not worth to add this kind of dependency management to this image. There are many tools for container deployment that can already take care of it.