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/

%s/digitalocean/netbox-community/gi ...

Thank you Digitalocean for sponsoring Netbox!
And thank you for letting it grow up and move out ;)

fix DEBUG

Consistent variable use in build scripts

Update references to netboxcommunity

Build system for hub.docker.com

✨ Prevent Github API Rate Limits ...

When building netbox-docker on Travis, it happened too often that the
build failed because of Github's API rate limits. These apply for all
requests, but if the request is unauthenticated they are applied by IP.

This is bad for netbox-docker, as we build on Travis where the build-
workers are shared with the rest of the world.

This commit adds the possibility to define OAuth credentials as
documented at [1]. Hopefully this leads to more reliable releases of
netbox-docker images.

[1] https://developer.github.com/v3/#increasing-the-unauthenticated-rate-limit-for-oauth-applications

Revert "⚙️ Print every command" ...

This reverts commit 1fb6b80.

⚙️ Print every command

⚙️ Additional build debug info

🐞 Fix error reporting

⚙️ Check variant when checking latest version

⚙️ Compact Variants Logic into build-all.sh

❇️ Adds DEBUG option

💄 Cleanup and hide warnings

Add django_ldap_auth ...

In the ldap.Dockerfile the django_ldap_auth module is installed to
enable authentication againt LDAP servers.

❇️ Don't build what's already there ...

This change introduces an API call to hub.docker.com to check if
the version we're about to build was already built & pushed. This
avoids superfluos builds.

✨ Only build prerelease if version higher than stable

✨ Build pre-released versions ...

... instead of development branches.

📦 First attempt to push Docker Images from Travis