chapter9: propose provide full path for --mount source

In the Disqus thread for chapter 9 (http://disq.us/t/4lw8u3p), reader
Dan Dao reports seeing error:
```
$ docker build -t superlists . && docker run -p 8888:8888 --mount type=bind,source=./src/db.sqlite3,target=/src/db.sqlite3 -it superlists
...
docker: Error response from daemon: invalid mount config for type "bind": invalid mount path: './src/db.sqlite3' mount path must be absolute.
```

I also encountered that error. Both of us could have done something
incorrect leading to the error.

I replaced
  source=./src/db.sqlite3
with
  source=${PWD}/src/db.sqlite3
to provide an absolute path.

and reran the command and did not get an error.

I have tried this only on:
macOS version 11.7.10
Docker version 20.10.14, build a224086

Author: zipperer

chapter_09_docker.asciidoc

@@ -1418,7 +1418,7 @@ and the `target` path _inside_ the container:
 ----
 $ *docker build -t superlists . && docker run \
   -p 8888:8888 \
-  --mount type=bind,source=./src/db.sqlite3,target=/src/db.sqlite3 \
+  --mount type=bind,source=${PWD}/src/db.sqlite3,target=/src/db.sqlite3 \
   -it superlists*
 ----