deps: update docker.io/library/alpine docker tag to v3.21.2

Author: renovate-bot

.cloudbuild/library_generation/library_generation.Dockerfile

@@ -27,7 +27,7 @@ RUN mvn install -B -ntp -DskipTests -Dclirr.skip -Dcheckstyle.skip
 RUN cp "/root/.m2/repository/com/google/api/gapic-generator-java/${DOCKER_GAPIC_GENERATOR_VERSION}/gapic-generator-java-${DOCKER_GAPIC_GENERATOR_VERSION}.jar" \
   "./gapic-generator-java.jar"
 
-FROM docker.io/library/alpine:3.21.1@sha256:b97e2a89d0b9e4011bb88c02ddf01c544b8c781acf1f4d559e7c8f12f1047ac3 as glibc-compat
+FROM docker.io/library/alpine:3.21.2@sha256:56fa17d2a7e7f168a043a2712e63aed1f8543aeafdcee47c58dcffe38ed51099 as glibc-compat
 
 RUN apk add git sudo
 # This SHA is the latest known-to-work version of this binary compatibility tool