Move docker build to before synapse make

This hopefully fixes the issue related to reading a file that travis
doesn't have read access to causing docker build to fail

.travis.yml

@@ -28,6 +28,8 @@ matrix:
         - COVERAGE=ON
 
 install:
+  if [ "$TRAVIS_OS_NAME" = "linux" ]; then docker build -t nheko-reborn/mtxclient -f Dockerfile . ; fi
+
   - ./.ci/install.sh
 
 script:
@@ -40,8 +42,6 @@ script:
   # Build the lib and run the test suite against synapse.
   - |
     if [ $TRAVIS_OS_NAME == linux ]; then
-      ls `pwd`
-      docker build -t nheko-reborn/mtxclient `pwd`/Dockerfile
       docker run -v `pwd`:/build --net=host \
         -e CXX_VERSION=${CXX_VERSION} \
         -e CC_VERSION=${CC_VERSION} \