Docker: Install ros-ign from Debian

[nkoenig]

Install ros-ign from debian instead of from source so that the ros image compression library is brought in.

[osrf-jenkins]

Build finished. No test results found.

[peci1]

Isn't this fix pretty fragile?

$ apt show ros-melodic-ros-ign 2>/dev/null | grep Depends
Depends: ros-melodic-ros-ign-bridge, ros-melodic-ros-ign-gazebo-demos, ros-melodic-ros-ign-image

$ apt show ros-melodic-ros-ign-gazebo-demos 2>/dev/null | grep Depends
Depends: libignition-gazebo4-dev, ros-melodic-image-transport-plugins, ros-melodic-ros-ign-bridge, ros-melodic-ros-ign-gazebo, ros-melodic-ros-ign-image, ros-melodic-rqt-image-view, ros-melodic-rqt-plot, ros-melodic-rviz

It only installs image transport plugins because ros-melodic-ros-ign-gazebo-demos depends on them. I guess that might never change, but why not install the plugins directly? Or why not explicitly use them as a dependency of ros-ign-image?