Use only one Dockerfile

[dunglas]

As both Dockerfiles are totally identical, let's use only one to ease the maintenance and reduce the build time.

[develroo]

In addition to this don't use ubuntu:20.4 it is failing.

Building nginx-relay
Step 1/8 : FROM ubuntu:20.04
20.04: Pulling from library/ubuntu
ERROR: Service 'nginx-relay' failed to build: no matching manifest for unknown in the manifest list entries

I changed it to debian:buster and it worked fine.

[dunglas]

In addition to this don't use ubuntu:20.4 it is failing.

I've not encountered this issue when playing with this project.