Dockerfiles: Rename working directory from "/build" to "/work"

Since the directory will contain both sources and build, this commit
generalizes its name.

imagefiles/dockcross

@@ -154,7 +154,7 @@ fi
 # Now, finally, run the command in a container
 #
 docker run -i -t --rm \
-    -v $PWD:/build \
+    -v $PWD:/work \
     $USER_IDS \
     $FINAL_ARGS \
     $FINAL_IMAGE "$@"