For reproducible builds, `python:3.7` isn't specific enough. python:3.7.3-alpine3.9 is more specific, for example. There aren't supposed to be breaking changes in the bugfix releases, but they'll happen anyway.
I could probably safely make it "python:3.7-alpine3.9" (instead of pinning to Python 3.7.3), since the issue was the Alpine version, but at this point I'm starting to really buy into the whole reproducible build thing.