When files are injected into an image via ADD in a Dockerfile, those files have the same UID/GID as what they had on the host. Recently, when building an image in a testing environment, this caused no problems. However, I then deployed it on a server, with a user that happened to have a different UID. This cause the application in the container to crash.

This problem is easy enough to avoid once you know about it, but it seems like a small detail of the host leaking into the container.

A solution I think would work would be to make the injected files assume the UID/GID of the user under which commands are running at that point in the Dockerfile. So by default it would be root, but if ADD is preceded by a USER command, the files would assume the ownership of the specified user.