Use the Docker Hub images for caching and keep the names the same were
applicable.

Instead of a persistent volume with the configuration, use environment
variables.

- Use a single volume for the config and data LDAP directories, easier
to keep in sync in different cases (backups, migration).
- Add backup script, save to backup volume.
- Allow passing the location of the SSL key and cert to enable SSL,
otherwise a self-signed certificate and key are generated with the FQDN
of the container.
- Drop the dockerfile-lint linter, didn't give any useful advice.
- Change base image to Debian Buster.
- Set stopsignal for the container.
- Allow setting the log level.
- Unset the cleartext root password environment variable.

- Name the slapd service as ldap (in case it's replaced by a different
LDAP server).
- Allow overriding the default environment variables (preparation for an
actual deployment).

- Use a patch to change the configuration files (instead of fiddling
with sed).
- Don't log the healthcheck requests.

- Replaced phpLDAPadmin with ldap-account-manager as the webui.
- Added nss-pam-ldapd as a test client.