use ccache to speed up CI builds

.travis.yml

 language: cpp
+cache: ccache
 
 matrix:
   include:
@@ -9,6 +10,7 @@ matrix:
           update: true
           packages:
             - bison
+            - ccache
             - ceres-solver
             - CMake
             - doxygen
@@ -26,7 +28,7 @@ matrix:
             - qt
             - yasm
       before_install:
-        - export PATH="/usr/local/opt/bison/bin:${PATH}"
+        - export PATH="/usr/local/opt/bison/bin:/usr/local/opt/ccache/libexec:${PATH}"
       script:
         - mkdir build
         - cd build
@@ -36,17 +38,26 @@ matrix:
       dist: xenial
       services:
         - docker
-      script:
+      before_script:
         - docker build --tag stitchem-cuda8:latest --file docker/ci.dockerfile --build-arg version=8 .
+        - docker create -v ${HOME}/.ccache:/ccache --name ccache stitchem-cuda8:latest
+      script:
+        - docker run --env CCACHE_DIR=/ccache --volumes-from ccache stitchem-cuda8:latest
     - name: CUDA 9 Linux
       dist: xenial
       services:
         - docker
-      script:
+      before_script:
         - docker build --tag stitchem-cuda9:latest --file docker/ci.dockerfile --build-arg version=9 .
+        - docker create -v ${HOME}/.ccache:/ccache --name ccache stitchem-cuda9:latest
+      script:
+        - docker run --env CCACHE_DIR=/ccache --volumes-from ccache stitchem-cuda9:latest
     - name: CUDA 10 Linux
       dist: xenial
       services:
         - docker
-      script:
+      before_script:
         - docker build --tag stitchem-cuda10:latest --file docker/ci.dockerfile --build-arg version=10 .
+        - docker create -v ${HOME}/.ccache:/ccache --name ccache stitchem-cuda10:latest
+      script:
+        - docker run --env CCACHE_DIR=/ccache --volumes-from ccache stitchem-cuda10:latest

docker/base.dockerfile

@@ -4,6 +4,7 @@ ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt update && apt install -y \
   bison \
+  ccache \
   doxygen \
   flex \
   git \

docker/ci.dockerfile

@@ -6,4 +6,4 @@ WORKDIR stitchEm
 RUN echo ${CUDA_SHORT} > cuda.version
 WORKDIR build
 RUN cmake -DCMAKE_BUILD_TYPE=Release -DLINUX_PKG=ON -DRTMP_NVENC=OFF -DCUDA_TARGET_ARCH="50" -G Ninja ..
-RUN ninja
+CMD ninja