feat(docker): introduce autoware-core stage

[youtalk]

Description

• refactor(docker): introduce src-imported stage #4709

This PR adds the autoware-core stage between the base and prebuilt stages.
This is a milestone for the multi-stage per-component builds mentioned in the discussion below. I will break down the prebuilt stage further in the next PR.
https://github.com/orgs/autowarefoundation/discussions/4661

The graph of the stage structure is shown below.

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

Interface changes

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[mergify]

⚠️ The sha of the head commit of this PR conflicts with #4713. Mergify cannot evaluate rules on this PR. ⚠️