Add custom requirements to docker compose

[Flix6x]

To do:

• Documentation regarding the possibility of adding custom requirements by adding a requirements.txt file to the flexmeasures-instance folder.
• Changelog entry.

[coveralls]

Pull Request Test Coverage Report for Build 3437492834

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.023%

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Totals
Change from base Build 3436864090:	0.0%
Covered Lines:	6562
Relevant Lines:	9474

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💛 - Coveralls

[nhoening]

Why is postgres-14 the one we should pin? It only happened to be the one you are using. I still think it's better than using the latest always, but this will then become outdated in a while. Maybe we need to think more about how we can pin down what FlexMeasures should support here and how to formalize that.

[Flix6x]

It's currently the most recent that didn't throw errors (for me) when running docker-compose up. Wouldn't it be preferable to be able to know exactly what postgres version users are running given a specific FlexMeasures version installed with docker?

[nhoening]

Maybe it's an edge case that you created data in your own installation, then want to run it in another, docker-based one. But maybe it'll happen more often.

I agree that being specific is a little better, but the drawback is that this needs to be kept up-to-date, the one with a version not.