chore(deps): update alpine docker tag to v3.21 (#372)

Author: renovate-bot

batch/cpp_application/application/Dockerfile

@@ -14,7 +14,7 @@
 
 # We chose Alpine to build the image because it has good support for creating
 # statically-linked, small programs.
-FROM alpine:3.20 AS build
+FROM alpine:3.21 AS build
 
 # Install the typical development tools for C++, and
 # the base OS headers and libraries.

batch/parallel/application/Dockerfile

@@ -14,7 +14,7 @@
 
 # We chose Alpine to build the image because it has good support for creating
 # statically-linked, small programs.
-FROM alpine:3.20 AS build
+FROM alpine:3.21 AS build
 
 # Install the typical development tools for C++, and
 # the base OS headers and libraries.

cloud-run-hello-world/Dockerfile

@@ -16,7 +16,7 @@
 # [START dockerfile]
 # We chose Alpine to build the image because it has good support for creating
 # statically-linked, small programs.
-FROM alpine:3.20 AS build
+FROM alpine:3.21 AS build
 
 # Install the typical development tools for C++, and
 # the base OS headers and libraries.