Android Dockerfile: add comments and try to shrink the image more

This image is downloaded on every Travis run. Making it smaller is important.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

scripts/docker/android-build-container/Dockerfile

@@ -19,15 +19,28 @@ RUN apt-get update  && \
     libglib2.0-0 \
     openjdk-8-jdk
 
+# create our working directory and place the local copies of the Qt
+# install, NDK and SDK there, plus the three files from the Subsurface
+# sources that we need to get the prep routines to run
 RUN mkdir -p /android
 ADD qt-opensource-linux-x64-5.*.run /android/
 ADD android-ndk-r*-linux-x86_64.zip /android/
+ADD sdk-tools-linux-*.zip /android/
 ADD android-build-wrapper.sh variables.sh qt-installer-noninteractive.qs /android/
 
-RUN cd /android && bash -x /android/android-build-wrapper.sh -prep-only && \
-    rm -f qt-opensource-linux-x64-*.run android-ndk-r*-linux-x86_64.zip && \
-    rm -rf android-sdk-linux/emulator Qt/[a-zA-Z]* && \
-    rm -rf $( find android-ndk*/platforms -name arch-mips -o -name arch-x86 ) && \
-    rm -rf android-ndk*/toolchains/x86-* android-ndk*/toolchains/llvm/prebuilt/x86-* && \
+# run the build wrapper in prep mode
+RUN cd /android && bash -x /android/android-build-wrapper.sh -prep-only
+
+# clean up the files that we don't need to keep the container smaller
+RUN cd /android && \
+    rm -rf qt-opensource-linux-x64-*.run \
+	   Qt/[a-zA-Z]* \
+           sdk-tools-linux-*.zip \
+	   android-ndk-r*-linux-x86_64.zip \
+           android-sdk-linux/emulator \
+           $( find android-ndk*/platforms -name arch-mips -o -name arch-x86 ) \
+           android-ndk*/toolchains/x86-* android-ndk*/toolchains/llvm/prebuilt/x86-* && \
     ls -l && \
     du -sh *
+RUN apt-get clean
+RUN touch /android/finished-`date`