This commit introduces a huge change in the build process.
What changed:
- Dockerfile.ldap was integrated into Dockerfile as a seperate
[build stage][multistage-build].
- All the build scripts were refactored according to this.
- The `docker-compose.yml` file was adjusted likewise.
- The main build script, `/build.sh`, now always builds all
targets (formerly called variants).
- The minimal requirements for Docker and docker-compose
have increased.
- The build on hub.docker.com must be adjusted.
This change should also fix#156 permanently.
[multistage-build]: https://docs.docker.com/develop/develop-images/multistage-build/
