Add '--no-install-recommends' in Dockerfile

It appears we can shave off some megabytes by including this satement
when installing packages. I have not run into any issues with this
yet, but this could potentially be a source of problems later.

src/Dockerfile

@@ -5,7 +5,7 @@ LABEL maintainer="Jonas Alfredsson <[email protected]>"
 RUN set -ex && \
 # Install all necessary packages.
     apt-get update && \
-    apt-get install -y \
+    apt-get install -y --no-install-recommends \
             build-essential \
             curl \
             libffi6 \