Container makes its data directory usable itself (closes #42)

The entrypoint now creates the data directory if it is missing and stops
the start when any step fails, so a failed cd can no longer change the
ownership of some other directory. It already took ownership of the
directory and dropped to the routewatch user; that is unchanged. The
README's upaas section now says only which path to mount.

Model: opus-5-5

README.md

@@ -235,9 +235,7 @@ with the goroutine count and Go memory figures.
 What the [upaas](https://git.eeqj.de/sneak/upaas) app needs:
 
 - Container port: `8080`.
-- Volume: one, at container path `/var/lib/berlin.sneak.app.routewatch`. upaas
-  does not create the host directory, so create it before the first deploy. The
-  entrypoint takes ownership of it, so a root-owned directory works.
+- Volume: one, at container path `/var/lib/berlin.sneak.app.routewatch`.
 - Environment: nothing is required. Leave `XDG_DATA_HOME`, `GOMEMLIMIT` and
   `MALLOC_ARENA_MAX` at the image's values. `DEBUG=routewatch` is optional and
   adds the `System stats` memory line to the log.