chore(deps): bump alpine from 3.18.4 to 3.21.2 in /docker

Bumps alpine from 3.18.4 to 3.21.2.

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

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

docker/Dockerfile

@@ -2,7 +2,7 @@ ARG TARGETPLATFORM
 
 # Need to use Alpine 3.18.4 which uses Node 18 for arm/v6 and arm/v7, otherwise the build hangs.
 # See https://github.com/nodejs/docker-node/issues/2077
-FROM alpine:3.18.4 AS linux-arm-alpine
+FROM alpine:3.21.2 AS linux-arm-alpine
 FROM alpine:3.21 AS linux-arm64-alpine
 FROM alpine:3.21 AS linux-amd64-alpine
 FROM alpine:3.21 AS linux-riscv64-alpine