build(deps): bump node in /docker/backend

Bumps node from 16.16.0-buster-slim to 16.20.2-buster-slim.

---
updated-dependencies:
- dependency-name: node
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

docker/backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:16.16.0-buster-slim AS builder
+FROM node:16.20.2-buster-slim AS builder
 
 ARG commitHash
 ENV MEMPOOL_COMMIT_HASH=${commitHash}
@@ -11,7 +11,7 @@ RUN apt-get install -y build-essential python3 pkg-config
 RUN npm install --omit=dev --omit=optional
 RUN npm run package
 
-FROM node:16.16.0-buster-slim
+FROM node:16.20.2-buster-slim
 
 WORKDIR /backend