docker: run as non-root, add HEALTHCHECK, document upaas deploy (closes #147)

The image had never been run. A trial run (fresh named volume, port 8080,
real targets, no notification endpoints) showed it exited at once: Viper
searches the working directory and, with a YAML config type, also matches an
extension-less file named dnswatcher, so the binary at /app/dnswatcher was
parsed as a config file. The binary now lives in /usr/local/bin and the
working directory is the data dir.

The runtime stage also gains an unprivileged dnswatcher user (uid 10001) that
owns /var/lib/dnswatcher, so a fresh named volume inherits writable ownership,
and a Docker HEALTHCHECK that probes /.well-known/healthcheck with busybox
wget. README gains a "Deploying with upaas" section.

Model: opus-4-8

Dockerfile

@@ -41,15 +41,31 @@ FROM alpine@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4
 
 RUN apk add --no-cache ca-certificates tzdata
 
-WORKDIR /app
+# The binary lives in /usr/local/bin, not the working directory: config
+# loading searches the working directory, and with a YAML config type Viper
+# also matches an extension-less file named "dnswatcher" there, so a binary
+# named "dnswatcher" in that directory would be parsed as a config file.
+COPY --from=builder /src/bin/dnswatcher /usr/local/bin/dnswatcher
 
-COPY --from=builder /src/bin/dnswatcher /app/dnswatcher
-
-# Create data directory
-RUN mkdir -p /var/lib/dnswatcher
+# Run as an unprivileged user. The data directory is owned by that user so
+# writes succeed both on a bind mount and when Docker seeds a fresh named
+# volume from the image (a fresh volume inherits this directory's ownership).
+RUN addgroup -S dnswatcher \
+    && adduser -S -G dnswatcher -u 10001 dnswatcher \
+    && mkdir -p /var/lib/dnswatcher \
+    && chown dnswatcher:dnswatcher /var/lib/dnswatcher
 
 ENV DNSWATCHER_DATA_DIR=/var/lib/dnswatcher
 
+WORKDIR /var/lib/dnswatcher
+
+USER dnswatcher
+
 EXPOSE 8080
 
-ENTRYPOINT ["/app/dnswatcher"]
+# busybox wget (already in alpine) probes the health endpoint. PORT defaults
+# to 8080 and is honoured if the operator overrides it.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+    CMD wget -q -O /dev/null "http://127.0.0.1:${PORT:-8080}/.well-known/healthcheck" || exit 1
+
+ENTRYPOINT ["/usr/local/bin/dnswatcher"]