src-cli: Unix user ID mismatch can cause permission errors

Created by: eseliger

When running a campaign with several steps, that use different images with varying user IDs, the shared volume attached to them will have different owner IDs on the files. This does not happen when

• using root containers only
• using the same UID in all containers

We should add some documentation around that limitation.

The problematic flow:

Stepping back, here's an example of the problematic flow: You run a campaign with a volume workspace. The first step runs as UID 1000 and runs yarn install, creating node_modules and friends as UID 1000. The second step runs as UID 2000 and runs yarn upgrade, which results in permission errors when it tries to write to node_modules. 💥