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.

TODO.md

@@ -28,6 +28,9 @@ The other open issue is https://git.eeqj.de/sneak/routewatch/issues/30.
 
 # Completed Steps
 
+- 2026-09-29: the entrypoint creates the data directory if it is missing and
+  stops the start if a step fails; README "Running under upaas" no longer
+  asks for the host directory to be created first (closes #42)
 - 2026-09-29: `.dockerignore` keeps `.git`, local build output, local
   databases and `.env` out of the Docker build context, and so out of the
   source archive in the image (closes #39)

entrypoint.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+set -euo pipefail
 
 # glibc silently ignores a malformed MALLOC_ARENA_MAX, so refuse it here.
 if [[ -n "${MALLOC_ARENA_MAX:-}" && ! "$MALLOC_ARENA_MAX" =~ ^[1-9][0-9]*$ ]]; then
@@ -6,6 +7,9 @@ if [[ -n "${MALLOC_ARENA_MAX:-}" && ! "$MALLOC_ARENA_MAX" =~ ^[1-9][0-9]*$ ]]; t
     exit 1
 fi
 
+# Give the data directory to the routewatch user before the daemon starts,
+# whether it is missing, an empty root-owned mount, or holds another uid's files.
+mkdir -p /var/lib/berlin.sneak.app.routewatch
 cd /var/lib/berlin.sneak.app.routewatch
 chown -R routewatch:routewatch .
 chmod 700 .