Refactor to multistage builds

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/

docker-compose.yml

@@ -1,10 +1,11 @@
-version: '3'
+version: '3.4'
 services:
   netbox: &netbox
     build:
-      context: .
-      args:
-        - BRANCH=${VERSION-master}
+     context: .
+     target: ${DOCKER_TARGET-main}
+     args:
+       - BRANCH=${VERSION-master}
     image: netboxcommunity/netbox:${VERSION-latest}
     depends_on:
     - postgres