Add Gitea Actions CI/CD and deploy descriptor

- .gitea/workflows/ci.yaml: test on PR/push to master, deploy on push to master
- deploy/docker-compose.yml: 15-puzzle container on the edge network with a persistent data volume
- Dockerfile: create /data owned by app for the DATA_FILE volume
- README: document deployment, required Actions secrets/variables, and persistence

Dockerfile

@@ -40,6 +40,12 @@ RUN apk add --no-cache tzdata ; \
     getent group ${GID} || addgroup --gid ${GID} ${TARGET_GROUP} ; \
     adduser -u ${UID} -g ${TARGET_GROUP} -D -S -h ${APPLICATION_HOME} ${TARGET_USER}
 WORKDIR ${APPLICATION_HOME}
+
+# Persisted game-data directory (DATA_FILE lives here). A named volume mounted
+# at /data inherits this ownership on first use, so the unprivileged app user
+# can write the temp-file + rename that repo.go performs.
+RUN mkdir -p /data && chown ${TARGET_USER}:${TARGET_GROUP} /data
+
 USER ${TARGET_USER}
 
 COPY --from=builder /tmp/bin/server /usr/bin/server