Bump Alpine Linux in Docker final stage

Commit 37c9838 ("Bump Alpine Linux for Docker image") updated
only the build stage but left the final stage unchanged. This commit
completes the version bump by upgrading the final stage as well.

Author: kaiii708

Dockerfile

@@ -10,7 +10,7 @@ COPY . .
 RUN make
 RUN make tool
 
-FROM alpine:3.19 AS final
+FROM alpine:3.21 AS final
 
 # copy in elf files
 COPY ./build/*.elf /home/root/rv32emu/build/