Removed duplicate code with multi-stage builds

[mbenlioglu]

Instead of having two separate Dockerfiles with only 1 line of difference, this PR combines them into one file. To build the images, we need to specify --target in the docker command, e.g.:

docker build --target bullseye -t steamcmd:bullseye -f bullseye/Dockerfile .
docker build --target bullseye-root -t steamcmd:bullseye-root -f bullseye/Dockerfile .

I believe this makes maintenance easier and overall better practice.

Also in this PR:

• Updated hardcoded versions of nano, curl and locales, because apt-get install is failing during build
• Removed wget. The only usage of it was to download steamcmd tarball, then it was removed with apt-get remove. Instead of that, I replaced wget -qO - with curl -fsSL which has the same desired effect.

[CM2Walki]

Good PR! In fact almost perfect PR, the only issue it had was that it broke Docker Hub's automated builds. I fixed that in #45. No worries, your contributions will of course show up in the repository even though this PR will be closed :).

Thanks for the contribution

[mbenlioglu]

Glad to help.