Fix docker build when oxidized gems are present

Wneh oxidized gems have been build in the current working directory, the
docker build may fail or install the wrong version of oxidized. Removing
the gems within the build resolves the problem.

Dockerfile

@@ -26,6 +26,9 @@ WORKDIR /tmp/oxidized
 # docker automated build gets shallow copy, but non-shallow copy cannot be unshallowed
 RUN git fetch --unshallow || true
 
+# Remove any older gems of oxidized
+RUN rm pkg/*
+
 # Ensure rugged is built with ssh support
 RUN rake build