chapter 9: docker run with --mount: restore ./src/db.sqlite3

Commit 85831c9 replaced `.`
with `${PWD}` because I (and another reader Dan Dao who posted
in the Disqus forum for the chapter) encountered 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.
```

This commit removes `${PWD}` and restores `.`

I encountered the error while using an outdated version of docker.
I have since used an updated version of docker and `.` does not
produce an error. So, this commit reverts the change from `.` to
`${PWD}`.

`${PWD}` remains a workaround for readers stuck with an old version of docker.

Author: zipperer

chapter_09_docker.asciidoc

@@ -1420,7 +1420,7 @@ and the `target` path _inside_ the container:
 ----
 $ *docker build -t superlists . && docker run \
   -p 8888:8888 \
-  --mount type=bind,source=${PWD}/src/db.sqlite3,target=/src/db.sqlite3 \
+  --mount type=bind,source=./src/db.sqlite3,target=/src/db.sqlite3 \
   -it superlists*
 ----