fix: container sets up its own data directory (closes #75)

bin/entrypoint.sh, still running as root, now creates DATA_DIR if
missing and gives it and /data to the netwatch user with mode 750
before starting the backend as that user. It stops the start instead
when a symbolic link is on the path to /data or DATA_DIR, since chown
and chmod would change what the link points to. An empty host
directory owned by root, or one holding files from another uid, works
with no step on the host, so the README no longer tells the operator
to create or chown it. The image no longer sets that ownership at
build time.

Model: opus-5-5

README.md

@@ -192,8 +192,9 @@ only inside the container, on `127.0.0.1:8081`. The image:
 - Sends the security headers `REPO_POLICIES.md` requires on every response, as
   `security-headers.conf` sets them, in place of the backend's own
 - Stores reports in `DATA_DIR`, `/data/reports` by default, on the `/data`
-  volume. The backend runs as user `netwatch` (uid 1000), so a directory
-  bind-mounted at `/data` must be writable by uid 1000
+  volume. Before the backend starts, the image creates `DATA_DIR` and gives it
+  and `/data` to user `netwatch` (uid 1000), which the backend runs as, so a
+  host directory bind-mounted at `/data` ends up owned by uid 1000
 - Writes buffered reports to disk on `docker stop`, and exits non-zero if nginx
   or the backend exits on its own, so the platform restarts it
 
@@ -203,16 +204,6 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
 
 - **Port:** container port `8080`.
 - **Volume:** container path `/data`; the reports are kept in `/data/reports`.
-- **First run:** upaas bind-mounts the host directory it is given and does not
-  create it, and the backend, which runs as uid 1000, does not start unless it
-  can write there. Create the directory, owned by uid 1000, before the first
-  deploy:
-
-    ```bash
-    mkdir -p /path/to/data
-    chown 1000:1000 /path/to/data
-    ```
-
 - **Environment variables:** none is required. An empty one counts as unset, and
   one set to a value netwatch cannot use stops the container at start, with the
   reason in its log.