Add pgAdmin to docker compose for nice local admin UI

[jamesgorrie]

Description

• adds pgAdmin to docker compose to give us a nice UI into the DB
  
• couple of yamllint fixes

Proposed version

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• Patch
• Minor version
• Major version

Type of change

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• Bug fix
• New feature
• Breaking change
• GitHub workflow update
• Documentation update
• Remove legacy code
• Dependency update

How Has This Been Tested?

This is a local change so can be tested by running make run and visiting http://localhost:8000

I'll be making some follow up changes, so I'll add some docs while there.

Reviewer Checklist

• DB_CLIENT DEPENDENCY IS ON THE LATEST VERSION
• The PR represents a single feature (small drive-by fixes are also ok)
• The PR includes tests that are sufficient for the level of risk
• The code is sufficiently commented, particularly in hard-to-understand areas
• Any required documentation updates have been made
• Any TODOs added are captured in future tickets
• No FIXMEs remain

[katybaulch]

Code quality checks are failing but otherwise happy -- following up from our chat just now in the office I think we might benefit from starting to write our own dockerfiles and store them in ECR -- as I imagine this might be reusable between repos :) Nice work!

[katybaulch]

lgtm, we probably want to fix the conflicting lint rules going forward though rather than ignoring