The image creates SWW_STATE_DIR when it is missing (closes #337)

Before `waf` starts, the image's `run` script now makes `SWW_STATE_DIR` if it is missing (`mkdir -p`), gives it to `sww:sww` and sets its mode to `0700`, as it already does for the cache directory's owner. A value that is not an absolute path, or an unreadable `SWW_STATE_DIR_FILE`, still changes nothing, and `waf` stops naming the setting. The `Dockerfile` comment, `docs/image.md`, `docs/settings.md` and `SPEC.md` say the image makes the directory. `script/example-app` checks that a missing `SWW_STATE_DIR` is made, owned by `sww` with mode `0700`.

- `chmod 0700` also applies to an existing directory, a mounted volume included, at every start of `waf`.

Model: opus-5-5

docs/image.md

@@ -19,7 +19,8 @@ way.
 - The service directory `/etc/service/sww`, with the `run` script
   `share/sww.run`.
 - `/var/lib/sww` for `waf`'s state files, and `/etc/sww/rules.d` with the
-  default rule file `share/rules.d/00-default.rules`.
+  default rule file `share/rules.d/00-default.rules`. The `run` script makes
+  `SWW_STATE_DIR` when it is missing.
 - `EXPOSE 8080`, and the `HEALTHCHECK` that runs `sww healthcheck`.
 
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