Dockerfile.pt-example - delete build or build-debug folders

This causes an issue, if you copy them over with COPY . . and you have
made a local build. It will then have existing fiels produced against
your local directory structure, which will not match what Docker based
build needs.

Dockerfile.pt-example

@@ -12,6 +12,7 @@ RUN apt-get update && \
 
 COPY . .
 
+RUN rm -rf build build-debug
 RUN make build-pt-example-debug
 
 CMD [ "./build-debug/bin/pt-example", "-n", "docker-pt-example" ]