Add tini to the user images

RStudio daemonizes itself when it starts, so it isn't being
passed the SIGTERM when the user pod stops. So it exits
uncleanly, causing corruption and other state issues. tini should
properly reap the rstudio processes during exit.

Ref berkeley-dsep-infra/datahub#2878
Ref berkeley-dsep-infra/datahub#2891

Dockerfile

@@ -27,6 +27,7 @@ RUN apt-get update -qq --yes && \
         tmux \
         wget \
         vim \
+        tini \
         build-essential \
         locales > /dev/null
 
@@ -62,3 +63,5 @@ RUN JUPYTER_DATA_DIR=${CONDA_DIR}/share/jupyter julia -e 'using Pkg; Pkg.add("IJ
 
 COPY install-julia-packages.jl /tmp/install-julia-packages.jl
 RUN /tmp/install-julia-packages.jl
+
+ENTRYPOINT ["/tini", "--"]