Bump and pin the base layer (#335)

Author: IsaacG

Dockerfile

@@ -11,7 +11,7 @@ RUN /usr/bin/composer install \
     --classmap-authoritative \
     --working-dir=/opt/representer
 
-FROM php:8.4-cli-alpine
+FROM php:8.4.21-cli-alpine3.23@sha256:4f4fc56fe4ba7b7d241c371eda011b27ca4f3b25bf2a37956ee06e966777d696
 
 COPY --from=builder /opt/representer /opt/representer