I'm using yarn berry, specifically version 3.0.0, but was also having the same issue with 2.4.2. When I run yarn install --immutable in CI I get the error:
[2021-07-27T04:51:41Z] ➤ YN0028: │ The lockfile would have been modified by this install, which is explicitly forbidden.
However when I run yarn install locally there are no changes to the yarn.lock file. I'm running the same node version 12.22.3 on my local machine and in the CI machine. The only other difference is that in CI, I'm running it inside an Alpine image whereas my local machine is a Mac, but I'm not reading that that should cause issues.
What can I do to debug this further?