Documenting known issue: pack 0.33.0 has breaking change for library users

[natalieparellano]

Description

This was reported in Slack: https://cloud-native.slack.com/archives/C033DV8D9FB/p1708014910021369

Previously, build.LifecycleOptions.UID didn't exist as a field / options for the "lifecycle executor".

When we merged #2017 we didn't realize that in order for the "lifecycle executor" to work properly the l.opts.UID must be set intentionally, to either:

• The CNB_USER_ID or some other positive value
• A negative value to avoid providing -uid to the lifecycle
• 0 if you really want to run the lifecycle as root (this will not work)

If l.opts.UID is unset (0) then a number of issues can occur:

• (trusted flow) The creator will error out at the detect phase, refusing to run as root
• (untrusted flow) The analyzer will chown the /layers directory to be root-owned, and the detector will fail to write group.toml there
• ...

Proposed solution

When upgrading to 0.33.0, pack library consumers MUST set build.LifecycleOptions.UID to either a positive or negative value. It cannot be 0.

Describe alternatives you've considered

We could roll back #2017. But, a similar requirement has already existed for some time for build.LifecycleOptions.GID.

[edmorley]

xref superfly/flyctl#3396