Separate yarn build and install in backend Dockerfile

[stanleegoodspeed]

Description

Move yarn build and yarn install to separate instructions

Checklist

Before you move on, make sure that:

• No unintended changes are included
• Spelling is correct
• There are tests covering new/changed functionality
• Commits have meaningful names and changes. CR remarks-like commits are squashed.
• Proper labels assigned. Use WIP label to indicate that state