Before this change, the docker user had home in a directory it had no
permissions on. The inability of creating a cache directory in `$HOME`
prevented npm to work properly.
Additionally, the `node_modules` in the base working directory had its
owner set to root, preventing further changes.
With this change, the `etherpad` user has a home directory.
Additionally, `npm i` is now run by `etherpad` rather than the root
user; this way, it is possible to dynamically change the `node_modules`
content in day 2 operations.
Note that while switching to the `useradd` builtin, a conflict was
discovered with the GID 65534 that was previously used. This change is
changing the `etherpad` user's UID to 5001 to avoid said conflict. As a
consequence, a `chmod -R 5001:5001` must be run prior to attaching
volumes created from previous Etherpad versions.
With this change, the Dockerfile builds the Docker image from the code
checked out in the local filesystem, instead of downloading a revision
from git.
Implements #3657
