added Dockerfile

[erwindon]

I hereby submit a plain Dockerfile.

hightlights:

• a debian-linux image is used
• the regular existing MVN build procedure is used
• the default hostname has been replaced with {{host.docker.internal}} as the original hostname {{localhost}} points to a location within the docker container. since we did not put a message broker there that is useless. when a broker is running on the same machine, it works. otherwise the actual broker still needs to be specified as usual.

typical use:
{{docker run imagename a --get queue1}}
In docker one must provide the whole actual command with its parameters as arguments on startup (but most common is that the predefined command is already good, then nothing need to be given). Since {{a}} is a client program, it always needs extra parameters to tell it what to do.

Note that a {{Dockerfile}} does not assign a label, so label {{imagename}} will be different. A typical label in DockerHub is {{a:latest}}.

[northlander]

closes #42

[northlander]

Did some minor changes to the Docker setup. Such as a multi stage build with alpine/jdk8 as final runtime image and a .dockerignore file to reduce size.

[erwindon]

@northlander
nice improvements!
from the initial issues description I could not derive your knowledge about Docker and assumed it was minimal. good to see that I was too pessimistic!