Provide a medium size image in whitehall image definitions #757

Job	Run time
Check content schemas are built	3m 16s
Test Ruby	2m 7s
Security Analysis / Run Brakeman	17s
Lint Ruby / Run RuboCop	20s
Run GDS API Adapter Pact tests / Verify pact tests	1m 12s
Run Content Store Pact tests / Verify pact tests	24s
Delete Pact artifact	4s
	7m 40s

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