Container sets its data directory's owner and mode itself (#353)

Closes #340.

The image no longer sets `USER`. Its new `ENTRYPOINT`, `deploy/docker-entrypoint.sh`, starts as root, creates `DATA_DIR` if missing, gives the directory and anything in it owned by another user to `webhooker` (UID 1000), sets the directory to `0750`, and runs the command as `webhooker` through `su-exec`. An empty root-owned bind mount, or data left by another UID, now works as mounted; the app never runs as root and is still PID 1. `CMD` is still `/app/webhooker`, so the `resetpw` commands are unchanged. Started with `--user`, the script only runs the command. It is in `/usr/local/bin`, not `/app`, which belongs to `webhooker`.

README: the UID 1000 ownership block, the upaas pre-deploy commands and the restore ownership step are gone; the upaas volume bullet names only the path.

- Judgement call: `su-exec` over `setpriv`: Alpine's small tool for this, needing only musl; busybox's `setpriv` cannot change user, and util-linux's adds `libcap-ng`.
- Deviation: `su-exec` is pinned by version (`0.2-r3`), not by hash; `ca-certificates` beside it is unpinned.
- Judgement call: each start reads every entry's owner but changes only entries owned by someone else.
- `docker exec` and the health check now run as root, since the image sets no `USER`.
- No automated test covers the script: the suite runs inside `docker build`, which cannot start a container.
- A missing host directory under upaas is sneak/upaas#235.

Model: opus-5-5
Reviewed-on: #353
Co-authored-by: clawbot <35+clawbot@noreply.example.org>

Dockerfile

@@ -88,7 +88,9 @@ RUN CGO_ENABLED=1 make build VERSION="$VERSION" GO_LDFLAGS='-extldflags "-static
 # alpine:3.21, 2026-03-17
 FROM alpine:3.21@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
 
-RUN apk --no-cache add ca-certificates
+# su-exec 0.2-r3 (Alpine 3.21), 2026-09-29: the entrypoint runs the app
+# as webhooker with it.
+RUN apk --no-cache add ca-certificates su-exec=0.2-r3
 
 # Create non-root user
 RUN addgroup -g 1000 -S webhooker && \
@@ -99,13 +101,17 @@ WORKDIR /app
 # Copy binary from builder
 COPY --from=builder /build/bin/webhooker /app/webhooker
 
+# Not under /app, which belongs to webhooker: this script runs as root.
+COPY deploy/docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
+
 # Create data directory for all SQLite databases (main app DB +
 # per-webhook event DBs). DATA_DIR defaults to /var/lib/webhooker.
 RUN mkdir -p /var/lib/webhooker
 
 RUN chown -R webhooker:webhooker /app /var/lib/webhooker
 
-USER webhooker
+# No USER: the entrypoint starts as root to make the data directory
+# webhooker's, then runs the app as webhooker.
 
 EXPOSE 8080
 
@@ -124,4 +130,5 @@ ENV BIND_ADDRESS=0.0.0.0
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
     CMD wget --no-verbose --tries=1 --spider http://localhost:8080/.well-known/healthcheck || exit 1
 
+ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
 CMD ["/app/webhooker"]