[seedspirit]

Summary

• The Alpine branch of entrypoint.sh creates users and groups with busybox adduser/addgroup, but adds the user to shadow and grpread with usermod -aG. busybox has no usermod, so on Alpine images without the shadow package (e.g. python:3.12-alpine) those three lines fail with usermod: not found and the user never joins the groups.
• Replace them with busybox addgroup <user> <group>, which does the same thing. Installing shadow at container start is avoided: it needs outbound network and slows every session start.
• The root-mode branch (usermod with an unset $USER_NAME) is a separate problem and is left untouched.

Test plan

Ran the whole script in python:3.12-alpine with LOCAL_USER_ID=501 LOCAL_GROUP_ID=501:

• New user path: before → usermod: not found ×2, no groups; after → groups=501(work),42(shadow),1002(grpread)
• Image already has a uid 501 user: before → usermod: not found; after → that user is in 42(shadow)
• Non-Alpine branch unchanged (diff is confined to the /bin/ash branch)

Resolves BA-8638

🤖 Generated with Claude Code

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