Add Dockerfile

[martialblog]

Hi,

this add a Dockerfile for the project so that users can run the exporter in a Container.

# amd64
docker build -t docker.io/just-an-example-dont-use-this/slurm-exporter .

#arm64
docker build --build-arg GOARCH=arm64 --build-arg=arm64v8 \ 
 -t docker.io/just-an-example-dont-use-this/slurm-exporter .                                       

docker run -ti --rm \
 -v /etc/slurm/slurm.conf:/etc/slurm/slurm.conf:ro \
 -v /etc/munge/munge.key:/etc/munge/munge.key:ro \
 -v /run/munge/munge.socket.2:/run/munge/munge.socket.2:ro\
 -p 8080:8080 docker.io/just-an-example-dont-use-this/slurm-exporter

Related to #76

[martialblog]

Let me know if I should also mention this in the README.