diff --git a/README.md b/README.md
index 0e34c82..dbf0e76 100644
--- a/README.md
+++ b/README.md
@@ -22,6 +22,15 @@ FROM ghcr.io/alphagov/govuk-ruby-base:3.0
 # your app image steps here
 ```
 
+## Common problems and resolutions
+
+`ERROR: failed to solve: cannot copy to non-directory: /var/lib/docker/overlay2/.../merged/app/tmp`
+
+Add `tmp/` to your `.dockerignore`. This happens because symlink `$APP/tmp` to
+`/tmp` as a workaround for some badly-behaved gems that assume they can write
+to `Path.join(Rails.root, 'tmp')` so that we can run with
+`readOnlyRootFilesystem`.
+
 
 ## Managing Ruby versions