Migrate DGU to Graviton/ARM

[dj-maisy]

Exactly as it says on the tin. Adapt DGU to be able to run on ARM and then run it on ARM.

Risks/Unknowns

• PostGIS - This may need have ARM support out of the box. Not a problem, now using Postgres natively.

Tasks

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1. Update Solr, Add --target to Dockerfile FROM layers ckanext-datagovuk#1777
   +0
2. Refactor Build Workflow for Multi-Arch Support ckanext-datagovuk#1797
   DGU enhancement +2