How to install PostGIS in Gel running in Docker?

[notramo]

I could not find anything in the documentation regarding PostGIS and Docker.

I want to enable PostGIS on a Gel instance running in Docker. Is it stored in the Docker data volume (alongside the table data), meaning that I can run the install command inside the container once, or is it stored alongside with the program files, meaning that I have to add the install command to the Dockerfile, to run it every time the container is built?

[zozzz]

I don't know which version it works from, but in nightly version you can define environment variable: GEL_DOCKER_EXTENSIONS=postgis