Do not bust the Docker cache (via `rosdep install`) when only sources have changed

[emersonknapp]

Description

When iterating on a workspace, any change that I make to the source (https://github.com/ros-tooling/cross_compile/blob/master/cross_compile/Dockerfile_ros#L100) busts the docker cache, and causes a full rosdep reinstall (https://github.com/ros-tooling/cross_compile/blob/master/cross_compile/Dockerfile_ros#L108) even if the needed dependencies haven't changed.

This rosdep install step can be one of the longest steps in the build process, especially for high level packages.

Completion Criteria

• Changes to the workspace source that do not change the dependencies should not trigger a full rosdep reinstall. Ideally, only the build step should be run if possible. This will tighten the development iteration loop.

Implementation Notes

• Potentially, can use a multi-stage Docker build
  • rosdep install --simulate outputs the installation commands that will be run, which can be dumped to a script
  • the sysroot creation step can run this script before taking the workspace sources - thus if a source change does not change the dependency list, the script will not change, and will not bust the cache

Testing Notes

• Be aware that caching can break in subtle ways
• Test pip-based rosdep dependencies
• Test that changing a package.xml dependency busts the rosdep cache
• The custom setup script / data directory can add rosdep rules, take this into account!
• You could parse docker output for "used cache" to test whether this worked properly

[thomas-moulard]

Punted from sprint #11 to sprint #12 on 2020-02-03