HADOOP-14366. Maven upgrade broke start-build-env.sh. Contributed by

Akira Ajisaka.

dev-support/docker/Dockerfile

@@ -93,6 +93,7 @@ RUN mkdir -p /opt/maven && \
          -o /opt/maven.tar.gz && \
     tar xzf /opt/maven.tar.gz --strip-components 1 -C /opt/maven
 ENV MAVEN_HOME /opt/maven
+ENV PATH "$PATH:/opt/maven/bin"
 
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 # Install findbugs