Fix NPM-installed files with large UID/GID.

For some unknown reasons (*cough* Javascript *cough*), when NPM's
`underscore` module is installed (as a dependency to Obographviz), its
files are given some impossibly large UID and GID. This may prevent the
final image from being used on systems where Docker runs in rootless
mode.

So after Obographviz is installed, we ensure that all files and
directories under /usr/local/lib/node_modules belong to root as they
should.

Dockerfile

@@ -88,7 +88,8 @@ RUN wget -nv http://archive.apache.org/dist/jena/binaries/apache-jena-$JENA_VERS
 RUN swipl -g "pack_install(sparqlprog, [interactive(false),global(true)])" -g halt
 
 # Install obographviz
-RUN npm install -g obographviz
+RUN npm install -g obographviz && \
+    chown -R root:root /usr/local/lib/node_modules
 
 # Install OBO-Dashboard.
 COPY scripts/obodash /tools