Fix Docker build command failing

[kbonnici]

This PR solves two issues:

1. The Python version in the old Dockerfile was tied to the latest version of the 3-alpine Docker image. This created an error in the docker build command where the Python version was no longer compatible with the version of Pillow. This issue gives a more detailed explanation of the problem. This PR locks the python version to 3.9.
2. The pip install layer of the Dockerfile fails because the image does not have the dependancies necessary to install everything in requirements.txt. This issue gives a more detailed explanation of the problem. This PR adds a layer in the Dockerfile to install these dependancies.

Closes #97 Closes #99