Add ccache to the base dockerfile.

Also organized what's being installed a bit.

docker/Dockerfile-base

@@ -1,8 +1,20 @@
 FROM ubuntu:16.04
 
 RUN apt update
+
 RUN apt install -y -qq apt-utils
-RUN apt install -y -qq libboost-filesystem1.58-dev libboost-filesystem1.58.0 libboost-iostreams1.58-dev libboost-iostreams1.58.0 libboost-locale1.58-dev libboost-locale1.58.0 libboost-regex1.58-dev libboost-regex1.58.0 libboost-serialization1.58-dev libboost-serialization1.58.0 libasio-dev libboost-program-options1.58-dev libboost-program-options1.58.0 libboost-random1.58-dev libboost-random1.58.0 libboost-system1.58-dev libboost-system1.58.0 libboost-thread1.58-dev libboost-thread1.58.0 libboost-test-dev libbz2-1.0 libbz2-dev bzip2 zlib1g zlib1g-dev libpangocairo-1.0-0 libssl-dev libsdl2-2.0-0 libsdl2-dev libsdl2-image-2.0-0 libsdl2-image-dev libsdl2-mixer-2.0-0 libsdl2-mixer-dev libsdl2-ttf-2.0-0 libsdl2-ttf-dev libpng12-0 libpng12-dev libreadline6-dev libvorbis-dev libcairo2 libcairo2-dev libpango-1.0-0 libpango1.0-dev libfribidi0 libfribidi-dev git scons gcc g++ make cmake clang openssl gdb xvfb python3
+
+# boost
+RUN apt install -y -qq libboost-filesystem1.58-dev libboost-filesystem1.58.0 libboost-iostreams1.58-dev libboost-iostreams1.58.0 libboost-locale1.58-dev libboost-locale1.58.0 libboost-regex1.58-dev libboost-regex1.58.0 libboost-serialization1.58-dev libboost-serialization1.58.0 libasio-dev libboost-program-options1.58-dev libboost-program-options1.58.0 libboost-random1.58-dev libboost-random1.58.0 libboost-system1.58-dev libboost-system1.58.0 libboost-thread1.58-dev libboost-thread1.58.0 libboost-test-dev
+
+# SDL
+RUN apt install -y -qq libsdl2-2.0-0 libsdl2-dev libsdl2-image-2.0-0 libsdl2-image-dev libsdl2-mixer-2.0-0 libsdl2-mixer-dev libsdl2-ttf-2.0-0 libsdl2-ttf-dev
+
+# misc
+RUN apt install -y -qq libpng12-0 libpng12-dev libreadline6-dev libvorbis-dev libcairo2 libcairo2-dev libpango-1.0-0 libpango1.0-dev libfribidi0 libfribidi-dev libbz2-1.0 libbz2-dev zlib1g zlib1g-dev libpangocairo-1.0-0 libssl-dev
+
+# programs
+RUN apt install -y -qq openssl gdb xvfb bzip2 git scons cmake make ccache gcc g++ clang
 
 WORKDIR /home/wesnoth-travis
 

docker/Dockerfile-travis

@@ -2,7 +2,5 @@ FROM wesnoth/wesnoth:16.04
 
 COPY ./ /home/wesnoth-travis/
 
-RUN apt install -y -qq ccache
-
 WORKDIR /home/wesnoth-travis