Backend build stage optimizations

[yuijim]

Small Dockerfile updates for the backend stage, aimed at reducing subsequent image builds (the initial build time is nearly the same).

Changes:

• Single apt install command at the beginning of the stage
• Copying necessary files & running the build setup in a separate step
• Copying & running the rest of the backend sources last

[wuhanstudio]

It's great to keep apt install in the beginning so that subsequent image builds can use the cache.

[mjxu96]

This change will be covered in this PR https://github.com/mjxu96/carlaviz/pull/62

Thanks!