fix: pin Docker base image

Description

Change foca image.

Fixes #102

[x] My code follows the style guidelines of this project
[x] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[x] My changes generate no new warnings
[ ] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes
[x] I have not reduced the existing code coverage
[x] I have added docstrings following the Python style guidelines of this project to all new modules, classes, methods and functions are documented with docstrings following; I have updated any previously existing docstrings, if applicable
[x] I have updated any sections of the app's documentation that are affected by the proposed changes, if applicable