build(Dockerfile): use FOCA v0.6.0 base image

[uniqueg]

Description

• Dockerfile now uses base image of FOCA v.0.6.0 (elixircloud/foca:20201114) rather than latest

Fixes #44

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] 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