Re-add unprivileged user to Dockerfile

Dockerfile

@@ -7,6 +7,9 @@ RUN cargo install --path .
 FROM alpine:latest
 RUN apk add --no-cache curl
 COPY --from=builder /usr/local/cargo/bin/libreddit /usr/local/bin/libreddit
+RUN adduser --system --home /nonexistent --no-create-home --disabled-password libreddit
+USER libreddit
 EXPOSE 8080
 HEALTHCHECK --interval=5m --timeout=3s CMD curl -f http://localhost:8080/settings || exit 1
+
 CMD ["libreddit"]