Simplify Dockerfiles.

[reitermarkus]

Everything should still be working as before, just a little bit DRYer.

[CM2Walki]

Hey @reitermarkus!
Your Dockerfiles fail to build. See this screencap for more:

Make sure to actually test your Dockerfiles before you submit them for a PR.

EDIT:
To be frank, I'm not a big fan of having a big entry script. Anything that can be run before and be part of the image, should remain in the Dockerfile.

I'm quiet intrigued by the PR, considering it gets rid of some of the duplication (which could be eliminated by having a proper base image for all 3 images). However, I chose to have these duplicates to prevent the creation of a additional image layer.

[reitermarkus]

Make sure to actually test your Dockerfiles before you submit them for a PR.

I did. The following commands run successfully:

docker build -t cm2network/csgo -f buster/Dockerfile .
docker build -t cm2network/csgo:sourcemod -f buster-sourcemod/Dockerfile .
docker build -t cm2network/csgo:metamod -f buster-sourcemod/Dockerfile .

to prevent the creation of a additional image layer

I think the number of layers is actually still the same in total.

[CM2Walki]

Closed by #26. Dockerfiles have been split into something more maintainable.