chore(docker): separate rosdep and colcon RUNs

[youtalk]

Description

This PR splits the rosdep install and colcon build RUN commands in the prebuilt stage. This slightly increases the size of the prebuilt and devel images, but allows the rosdep install cache to work better when only modifying source code.

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

Interface changes

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