Include gdb and strace in Blimp Engine container image

blimp/engine/Dockerfile

 FROM ubuntu:trusty
 
 # Run the command below to update the lib list.
 # ldd ./blimp_engine_app | grep usr/lib | awk '{print $3}' | xargs -n1 \
 #   dpkg-query -S | awk -F: '{print $1}' | sort | uniq
 RUN apt-get update && \
   apt-get install -yq libdrm2 libfontconfig1 libfreetype6 libgraphite2-3 \
   libharfbuzz0b libnspr4 libnss3 libstdc++6
 
+# stunnel4 is needed for incoming TLS connections.
+# wget is needed for crash reporting.
 RUN apt-get update && apt-get install -yq stunnel4 wget
 
+# gdb and strace are not strictly required, but having them around makes
+# debugging easier.
+RUN apt-get update && apt-get install -yq gdb strace

[Kevin M, 2016/06/09 21:10:14]

Why are we updating so many times? One update at the beginning should suffice.
Every unnecessary update will add several seconds' delay.

[maniscalco, 2016/06/09 23:16:54]

Since the impact occurs when someone manually invokes docker build and not part
of a ninja build (or on the build bot) I wasn't super worried about an increase
in "docker build" execution time.

I just now tested on my workstation.  I ran both a couple times and ignored the
first runs (which took on the order of minutes).  For whatever reason, it was
actually faster with the extra updates on subsequent "hot" runs (34 seconds vs
36).  Who knows why... it's so close that maybe the difference is just noise?

Anyway, the benefit of including the updates is that each RUN line is
independent of the others.

LMK if you feel strongly about changing this.

[Kevin M, 2016/06/09 23:25:11]

I'm not sure if having these lines be independent is a big win since the lines
are meant to be executed from top to bottom... but I don't feel strongly about
this either way.

lgtm

+
 RUN mkdir /engine
 
 RUN useradd -ms /bin/bash blimp_user
 
 # The glob below expands to all files, but does not add directories
 # recursively.
 ADD * /engine/
 ADD gen/third_party/blimp_fonts /engine/fonts
 RUN mv /engine/chrome_sandbox /engine/chrome-sandbox
 RUN chown -R blimp_user /engine
 
 USER blimp_user
 WORKDIR "/engine"
 
 ENTRYPOINT ["/engine/start_engine.sh"]