Update alpine Docker tag to v3.21.3 (#123)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Dockerfile

@@ -18,7 +18,7 @@ RUN touch /usr/src/cloudflare-ddns/src/main.rs
 RUN cargo build --target x86_64-unknown-linux-musl --release
 
 ##### Runtime
-FROM alpine:3.21.2 AS runtime
+FROM alpine:3.21.3 AS runtime
 
 # Copy generated binary in runtime
 COPY --from=builder /usr/src/cloudflare-ddns/target/x86_64-unknown-linux-musl/release/cloudflare-ddns /usr/local/bin