The docker image is rebuilt when one of its ancestors is rebuilt, and when that happens, it fails with:
copying src/cryptography/py.typed -> build/lib.linux-x86_64-3.7/cryptography
generating cffi module 'build/temp.linux-x86_64-3.7/_padding.c'
generating cffi module 'build/temp.linux-x86_64-3.7/_openssl.c'
If you are seeing a compilation error please try the following steps to
successfully install cryptography:
1) Upgrade to the latest pip and try again. This will fix errors for most
users. See: https://pip.pypa.io/en/stable/installing/#upgrading-pip
2) Read https://cryptography.io/en/latest/installation.html for specific
instructions for your platform.
3) Check our frequently asked questions for more information:
4) Ensure you have a recent Rust toolchain installed:
5) If you are experiencing issues with Rust for *this release only* you may
set the environment variable `CRYPTOGRAPHY_DONT_BUILD_RUST=1`.
error: Can not find Rust compiler
This is due to the recent change in the python cryptography module, that now has a dependency on rust. It wouldn't be a problem if they provided wheels for python 3.7, but they don't.