Remove build deps from minimal-kernel image

* Remove SHELL env var and leftover USER nobody
* Install wget for tini then immediately uninstall it
* Install build-essentials and python3-dev for tornado speed up
  then immediately uninstall them

(c) Copyright IBM Corp. 2016

minimal-kernel/Dockerfile

@@ -8,21 +8,22 @@ USER root
 # Install all OS dependencies for fully functional notebook server
 ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update && apt-get install -yq --no-install-recommends \
-    wget \
-    python3-dev \
-    build-essential \
     python3-pip \
     python3-zmq \
     && apt-get clean
 
 # Install Tini
-RUN wget --quiet https://github.com/krallin/tini/releases/download/v0.6.0/tini && \
+RUN apt-get update && \
+    apt-get install -yq --no-install-recommends wget && \
+    wget --quiet https://github.com/krallin/tini/releases/download/v0.6.0/tini && \
     echo "d5ed732199c36a1189320e6c4859f0169e950692f451c03e7854243b95f4234b *tini" | sha256sum -c - && \
     mv tini /usr/local/bin/tini && \
-    chmod +x /usr/local/bin/tini
+    chmod +x /usr/local/bin/tini && \
+    apt-get remove --purge -y wget && \
+    apt-get autoremove -y && \
+    apt-get clean
 
 # Configure environment
-ENV SHELL /bin/bash
 ENV KG_USER jovyan
 ENV KG_UID 1000
 
@@ -30,14 +31,18 @@ ENV KG_UID 1000
 RUN useradd -m -s /bin/bash -N -u $KG_UID $KG_USER
 
 # Install Kernel Gateway
-RUN pip3 install jupyter_kernel_gateway==0.2.0
+RUN apt-get update && \
+    apt-get install -yq --no-install-recommends build-essential python3-dev && \
+    pip3 install jupyter_kernel_gateway==0.2.0 && \
+    apt-get remove --purge -y build-essential python3-dev && \
+    apt-get autoremove -y && \
+    apt-get clean
 
 # Configure container startup
-USER nobody
 EXPOSE 8888
 WORKDIR /tmp
 ENTRYPOINT ["tini", "--", "jupyter", "kernelgateway"]
 CMD ["--KernelGatewayApp.ip=0.0.0.0"]
 
 # Run container as jovyan
-USER jovyan
\ No newline at end of file
+USER jovyan