add openssh-client to enable git clone via ssh

I like to mount my .ssh and .gitconfig folders inside of containers, and was surprised to find myself unable to `git clone git@github.com: ...`, this addresses that. Adds about 7MB to docker image.

.build/Dockerfile

@@ -41,6 +41,7 @@ RUN apt-get update \
     locales \
     fonts-liberation \
     run-one \
+    openssh-client \
  && apt-get clean && rm -rf /var/lib/apt/lists/*
 
 RUN echo "en_US.UTF-8 UTF-8" > /etc/locale.gen && \