Update alpine Docker tag to v3.21

Author: renovate[bot]

apps/bar/Dockerfile

@@ -8,7 +8,7 @@ RUN cd /src && \
     go build -o example-app
 
 # Build the final image
-FROM alpine:3.18 as final
+FROM alpine:3.21 as final
 
 # Install the cloudposse alpine repository
 ADD https://apk.cloudposse.com/[email protected] /etc/apk/keys/

apps/foo/Dockerfile

@@ -8,7 +8,7 @@ RUN cd /src && \
     go build -o example-app
 
 # Build the final image
-FROM alpine:3.18 as final
+FROM alpine:3.21 as final
 
 # Install the cloudposse alpine repository
 ADD https://apk.cloudposse.com/[email protected] /etc/apk/keys/