If apt install docker doesn't work, you have to either use apt install docker.io or e.g. not use apt at all and install Docker "officially" from docker.com
Also, using snap to install Docker is not advised sudo snap install docker gives, an [Errno 13] Permission denied error message (for python3.11-minimal) and this blocks creating the Docker image.
(also, some may have to run sudo setfacl -R -m u:$USER:rwx /var/run/docker.sock first, if Docker wasn't installed from docker.com)
Add all desired languages for the build process.
After that you can build the docker image using:
make f Makefile.docker image # Builds the Docker image.
Then build the rom using;
make -f Makefile.docker
make -f Makefile.docker german_audio
make -f Makefile.docker french_audio
make -f Makefile.docker russian_audio
make -f Makefile.docker spanish_audio
make -f Makefile.docker # Run this after running the commands for the desired languages that you would like to add to your ROM.
Make sure to put all the following files in the portal64/vpk folder then run:
make -f Makefile.docker all_languages
If you have issues use make -f Makefile.docker clean to clean out any previous build files, remember it also removes any languages you set up so you will need to run those commands again.
make -f Makefile.docker clean
That will generate the rom at /build/portal64.z64
If you run Docker in sudo you may want to change the permissions of portal64/build, portal64/portal_pak_dir and portal64portal_pak_modified:
sudo chmod 777 -R build
sudo chmod 777 -R portal_pak_dir
sudo chmod 777 -R portal_pak_modified