Warning about missing rosdep update

[mabelzhang]

This is more an inconvenience than a bug.

The current Dockerfile doesn't run rosdep update, so when you run something, you get a warning like this:

$ roslaunch dave_demo_launch dave_dvl_gradient_demo_dsl.launch
…
the rosdep view is empty: call 'sudo rosdep init' and 'rosdep update'

That has to be manually run in the container right now.

One fix is to add a line like this in the Dockerfile:

RUN rosdep init && rosdep update

or this (actually the rosdep init errors saying a file in /etc already exists, maybe because I'm building from cached instead of scratch):

RUN rosdep update

But that's not optimal, because building the image will then warn that you aren't supposed to run it as the root user

Warning: running 'rosdep update' as root is not recommended.
  You should run 'sudo rosdep fix-permissions' and invoke 'rosdep update' again without sudo.

We can't get around that in the Dockerfile, since the Dockerfile only has a root user, and the non-root user is established using rocker.

A few options:

1. Do nothing, user will just have to run rosdep init && rosdep update manually in the container
2. Add RUN rosdep init && rosdep update to the Dockerfile, which will run as root user. Ignore the warning.
3. Add a non-root user to the Dockerfile and the RUN line above. This might conflict with the --user flag to rocker.