docker: Install argp.h

The 5.10 build fails with:

  CC [U]  kernel/bpf/preload/iterators/iterators.o
kernel/bpf/preload/iterators/iterators.c:3:10: fatal error: argp.h: No such file or directory
    3 | #include <argp.h>
      |          ^~~~~~~~
compilation terminated.

Fix by installing the required header.

Author: danobi

docker/Dockerfile.kernel

@@ -2,6 +2,7 @@ ARG ALPINE_VERSION
 FROM alpine:${ALPINE_VERSION}
 
 RUN apk update && apk add \
+  argp-standalone \
   bash \
   bison \
   build-base \