Shrink codespace

## What I did
Enabled prebuilt codespaces, to make the codespace start up much faster.

## What happened
The storage space used up my free monthly quota in less than a month.

## What could happen
If the docker image were smaller, maybe based on a slim Debian image, then it wouldn't use up the quota so fast.

Locate the Docker image configuration used by prebuilt Codespaces and inspect its base image and installed dependencies. Compare the resulting image size and verify that prebuilt Codespaces still start successfully with the needed project tooling.

Written by the indexing model from the issue text.