docker: fix Python cryptography build failure

docker: fix Python cryptography build failure

It seems like recent Python cryptography package versions need Rust
compiler present for OpenSSL wrapper, which leads to following error:

 generating cffi module 'build/temp.linux-x86_64-3.7/_openssl.c'
 running build_rust
 ...
 error: Can not find Rust compiler

There is following note regarding this issue in the documentation:

 If you are on RHEL/CentOS/Fedora/Debian/Ubuntu or another distribution
 derived from the preceding list, then you should upgrade pip (in a
 virtual environment!) and attempt to install cryptography again before
 trying to install the Rust toolchain. These platforms will receive a
 binary wheel and require no compiler if you have an updated pip!

So fix this by recommended pip update.

References: https://cryptography.io/en/latest/installation.html#rust
Signed-off-by: Petr Štetiar <ynezz@true.cz>