Build Python3.7 images from Alpine 3.10 instead of 3.11

[tobybellwood]

This PR forces Lagoon to use Alpine 3.10 to build Python 3.7 images instead of 3.11

This is because Alpine 3.11 defaults to Python 3.8, and as such will install the 3.8 version of Python3-dev (and Python3.8 as well to match the dependency), resulting in version duality. First discovered in testing #1887

For a later PR should be a rework of the Python images - there is a lot of optimisation that can be done - we may not need to augment the upstream images, or may be better using a glibc-based base image (e.g. buster-slim)

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for changelog and subsystem label(s) applied

Closing issues

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