TT#17502 Dockerfile: update documentation

* remove downloaded debs

Change-Id: Ib4ad0d594bc71c2dd8009db4c22d2f3b1075039b

t/Dockerfile

@@ -17,18 +17,20 @@ RUN pip install -r /tmp/test.txt
 
 RUN echo './t/testrunner' >>/root/.bash_history
 
+RUN apt-get clean
+
 WORKDIR /code/
 
 ################################################################################
 # Instructions for usage
 # ----------------------
 # When you want to build the base image from scratch (jump to the next section if you don't want to build yourself!):
-# % docker build --tag="repoapi-jessie" .
-# % docker run --rm -i -t -v $(pwd)/..:/code:rw repoapi-jessie:latest bash
+# % docker build --tag="repoapi-jessie" -f t/Dockerfile .
+# % docker run --rm -i -t -v $(pwd):/code:rw repoapi-jessie:latest bash
 #
 # Use the existing docker image:
 # % docker pull docker.mgm.sipwise.com/repoapi-jessie
-# % docker run --rm -i -t -v $(pwd)/..:/code:rw docker.mgm.sipwise.com/repoapi-jessie:latest bash
+# % docker run --rm -i -t -v $(pwd):/code:rw docker.mgm.sipwise.com/repoapi-jessie:latest bash
 #
 # Inside docker (the command is in history, just press UP button):
 #   ./t/testrunner