- hadolint and docker-compose config.
- Fix what hadolint complained about.

- 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).

- Specify the `/run/slapd` volume in the Dockerfile.
- Document TCP ports and Unix socket for slapd.

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

network to communicate with other containers.

- ldap-account-manager is not available on Debian buster right now, use
Debian sid instead.
- Document the default password.

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

- Allow setting a debug level environment variable.
- Expose port 636 (ldaps).
- Install newer version of slapd from Debian backports.
- Cleaned up the entrypoint script.