refactor(docker): introduce src-imported stage

[youtalk]

Description

This PR adds the src-imported stage to the top stage of the Dockerfile. This allows the base and src-imported stages to be processed in parallel, reducing the download latency of the vcs import command. It also eliminates the need to run the vcs import command again in the runtime stage. The trial in my local environment speeded up the docker build by about 5 minutes.

This PR does not cut out rosdep install, based on the reflection of the previous PR: https://github.com/autowarefoundation/autoware/pull/4656#issuecomment-2082095239

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

Interface changes

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[youtalk]

@mitsudome-r Please merge it because I have no longer the write access anyhow.

[xmfcx]

How was this tested?

I can merge it but I'm worried it might break the containers since I didn't test it.

[youtalk]

@xmfcx How can I test the container? I'm sorry but I don't know the test function since I'm new.