scaffold: aports overlay, build script, CI workflow, README

Kernel aport forked from pmaports 7.1.2-r0, repointed at milos-linux
combined-stable with the tested FP6 config (+EFI_ZBOOT for packaging);
imsd + nftables aports from the pending pmaports submission. build.sh
carries TODO(validate) markers - not yet run end-to-end.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

aports/main/postmarketos-config-nftables/rules/51_docker.nft

@@ -0,0 +1,11 @@
+#!/usr/sbin/nft -f
+table inet filter {
+	chain input {
+		iifname "docker*" accept comment "Allow incoming network traffic from Docker"
+	}
+
+	chain forward {
+		iifname "docker*" accept comment "Allow outgoing network traffic from Docker"
+		ct state {established, related} counter accept comment "accept established connections"
+	}
+}