Emphacise need to pre-create config.toml

The README did not make it clear enough that a blank `config.toml` file
needs to be pre-created before runner registration.  Otherwise the
`register` *runlabel* will silently fail to bind-mount the file into the
proper location inside the container.

While we're at it, also add a small example of why it may be a good idea
to edit the `config.toml` file before commiting it as a secret.

Signed-off-by: Chris Evich <cevich@redhat.com>

Containerfile

@@ -43,7 +43,7 @@ ARG NESTED_PRIVILEGED=true
 # may be changed if you know what you're doing.
 LABEL register="podman run -it --rm \
  --secret=REGISTRATION_TOKEN,type=env \
- -v ./config.toml:/home/runner/.gitlab-runner//config.toml:Z \
+ -v ./config.toml:/home/runner/.gitlab-runner/config.toml:Z \
  -e REGISTER_NON_INTERACTIVE=true \
  -e CI_SERVER_URL=https://gitlab.com/ \
  -e RUNNER_NAME=pipglr \

README.md

@@ -60,6 +60,15 @@ refers to this value as an *activation* token, but the usage is the same.
 ```bash
 $ IMAGE="registry.gitlab.com/qontainers/pipglr:latest"
 $ echo '<actual registration token>' | podman secret create REGISTRATION_TOKEN -
+```
+
+Next, ***a blank `config.toml` file*** needs to be created.  Without this, the
+`reigster` *runlabel* will return a permission-denied error.  Once the empty
+`config.toml` file is created, you may register one or more runners by repeating
+the registration *runlabel* as follows:
+
+```bash
+$ IMAGE="registry.gitlab.com/qontainers/pipglr:latest"
 $ touch ./config.toml  # important: file must exist, even if empty.
 $ podman container runlabel register $IMAGE
 ...repeat as desired...
@@ -80,6 +89,11 @@ $ podman secret create config.toml ./config.toml
 $ rm ./config.toml  # if desired
 ```
 
+This may be necessary, for example, to increase the default `concurrency` value
+to reflect the number of registered runners.  If you need to edit this file
+after commiting it as a secret, there's
+[ a `dumpconfig`  *runlabel* for that](README.md#configuration-editing).
+
 #### Volume setup (step 3)
 
 Since several users are utilized inside the container volumes must be