Update Dockerfile to build cryptography > 3.4

[djds]

• https://cryptography.io/en/3.4.5/installation.html#building-cryptography-on-linux
• https://github.com/pyca/cryptography/blob/main/CHANGELOG.rst#34---2021-02-07
• https://github.com/pyca/cryptography/blob/main/CHANGELOG.rst#3500---2021-09-29
• https://github.com/pyca/cryptography/issues/5771

[dek443]

Tested, and working as expected. Thank you for your continuous contributions and supporting the Dshell project.