backend/docker: bump library/node in /backend

Bumps library/node from 18-alpine3.18 to 21-alpine3.18.

---
updated-dependencies:
- dependency-name: library/node
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/Dockerfile

@@ -27,7 +27,7 @@ COPY ./build-gops.sh .
 RUN --mount=target=/root/.cache,type=cache --mount=target=/go/pkg/mod,type=cache \
     ./build-gops.sh
 
-FROM docker.io/library/node:18-alpine3.18@sha256:435dcad253bb5b7f347ebc69c8cc52de7c912eb7241098b920f2fc2d7843183d
+FROM docker.io/library/node:21-alpine3.18@sha256:4a512d1538b1a8281b58cab0b366a5c62436566bb63e7dcd4a6769c98edb3b5f
 # TARGETOS is an automatic platform ARG enabled by Docker BuildKit.
 ARG TARGETOS
 # TARGETARCH is an automatic platform ARG enabled by Docker BuildKit.