Upgrade project structure so a package can be build (#244)

Author: javierdelapuente

.dockerignore

@@ -5,3 +5,4 @@
 !/main.py
 !/requirements.txt
 !/src
+!/pyproject.toml

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- Project prepared to be installed with `pip install`, so it can be reused in 
+  the repository https://github.com/canonical/gatekeeper-repo-test
+
 ## [v0.9.0] - 2024-04-04
 
 ### Added