PIL requires a string format name, whereas `Format.PNG` is an "enum string"
which is not quite the same type.
I had not noticed this problem because I had manually tested the test on a VM
where the cached Stable Diffusion XL Docker image was from before this change.
Tested on a fresh VM with a freshly-built Docker image now.
This is split in two changes because the image needs to be rebuilt.
The first change updates the image, and the second change re-enables the test.
PiperOrigin-RevId: 618048266
Etienne PerotandgVisor bot