fix terminology EXPOSE -> port mapping

Author: Xronophobe

chapter_09_docker.asciidoc

@@ -1073,7 +1073,10 @@ so the ports _inside_ the container are different
 from the ports _outside_ the container, the ones we can see on our host machine.
 
 So we need to tell Docker to connect the internal ports to the outside ones,
-or "expose" them, in Docker terminology.
+to "publish" or "map" them, in Docker terminology.
+// CSANAD:  "expose" is different, it's for inter-container communication and it's
+//          done by the EXPOSE instruction in the Dockerfile. The -p is short for
+// --publish and this is port mapping.
 
 `docker run` takes a `-p` argument, with the syntax `OUTSIDE:INSIDE`.
 So you can actually map a different port number on the inside and outside.
@@ -1112,7 +1115,7 @@ curl: (52) Empty reply from server
 
 ==== Essential Googling the Error Message
 
-The need to expose ports and the `-p` argument to `docker run` are something you just learn,
+The need to map ports and the `-p` argument to `docker run` are something you just learn,
 fairly on in learning Docker.  But the next debugging step is quite a bit more obscure
 (although admittedly Itamar does address it in his
 https://pythonspeed.com/articles/docker-connection-refused/[docker networking article],