Shrink image another 20MB with separate node_modules ci --omit=dev stage

[rmlearney-digicatapult]

Pull Request

Checklist

• Have you read Digital Catapult's Code of Conduct?
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

PR Type

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• Chore

Linked tickets

High level description

Reduce docker image further 20MB to 264MB

Detailed description

Using separate layer to install node_modules with ci --omit=dev and copying into final production image prevents unnecessary transfer of cache

Describe alternatives you've considered

Operational impact

Additional context