What did you do?

With the following Dockerfile:

FROM python:3.6-slim
RUN pip install --no-cache Pillow==4.0.0
RUN find /usr/local/lib/python3.6/site-packages -name '*.so*' | xargs ldd

Run docker build -t pillow-ldd .. Observe output of last RUN command.

What did you expect to happen?

Expected all libraries to be linked successfully.

What actually happened?

All libraries are linked except for libpng:

/usr/local/lib/python3.6/site-packages/PIL/.libs/libpng16-4b619e46.so.16.25.0:
	linux-vdso.so.1 (0x00007ffc46363000)
	**libz-a147dcb0.so.1.2.3 => not found**
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f78e2e06000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f78e2a5a000)
	/lib64/ld-linux-x86-64.so.2 (0x00005652f53d8000)

Other libraries link to libz-a147dcb0.so.1.2.3 successfully, just not libpng for some reason.

I don't know if this manifests in any actual issues. It's just concerning that the library doesn't link.

What versions of Pillow and Python are you using?

As you can see, this is Python 3.6 with Pillow 4.0.0. I've seen the same with Python 2.7 + Pillow 4.0.0.