Create .env configuration file from example file:

$ cp example.env .env

Build image with HLDS Server. It includes raw Steam HLDS + Metamod + DProto + AMX MOD X:

$ docker-compose build hlds

Build custom server image based on hlds image. It includes maps, configuration and the other customization stuff:

$ docker-compose build server

Edit .env configuration variables if needed (see image entrypoint file hlds_run.sh if you want to figure out how variables are used).

Start your server:

$ docker-compose up -d server

Enjoy ;)