Add a development eBPF container build

See `docs/ebpf.md` for how to use. This is built by CI or you can build
manually if you customise the kernel.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

alpine/base/ebpf/decl.patch

@@ -0,0 +1,23 @@
+--- libelf/elf.h-orig
++++ libelf/elf.h
+@@ -21,7 +21,9 @@
+ 
+ #include <features.h>
+ 
+-__BEGIN_DECLS
++#ifdef __cplusplus
++extern "C" {
++#endif
+ 
+ /* Standard ELF types.  */
+ 
+@@ -3553,6 +3555,8 @@
+ #define R_TILEGX_NUM		130
+ 
+ 
+-__END_DECLS
++#ifdef __cplusplus
++}
++#endif
+ 
+ #endif	/* elf.h */