Fixed test setup for Docker container

.travis.sh

@@ -51,10 +51,9 @@ docker build -t $TRAVIS_REPO_SLUG .
 # Prepare build regression test
 pushd /tmp
 mkdocs new test && cd test
-echo "theme: \n  name: material" >> mkdocs.yml
 
 # Test Docker image build
-docker run --rm -it -v `pwd`:/docs $TRAVIS_REPO_SLUG build
+docker run --rm -it -v `pwd`:/docs $TRAVIS_REPO_SLUG build --theme material
 
 # Return to original directory
 popd

Dockerfile

@@ -33,7 +33,7 @@ COPY setup.py setup.py
 
 # Perform build and cleanup artifacts
 RUN \
-  apk add -U openssh git && \
+  apk add --no-cache openssh git && \
   python setup.py install 2>/dev/null && \
   rm -rf /tmp/*