automate snapshot date bump

Makefile

@@ -119,11 +119,15 @@ update-packages:
 		--user $(userid):$(groupid) \
 		--volume $(PWD)/config:/home/build/config \
 		--volume $(PWD)/scripts:/home/build/scripts \
+		--env GIT_EPOCH="$(GIT_EPOCH)" \
 		$(IMAGE) tail -f /dev/null
 	$(docker) exec -it --user=root "$(NAME)" update-packages
 	$(docker) cp \
 		"$(NAME):/etc/apt/packages.list" \
 		"$(PWD)/config/container/packages.list"
+	$(docker) cp \
+		"$(NAME):/etc/apt/sources.list" \
+		"$(PWD)/config/container/sources.list"
 	$(docker) rm -f "$(NAME)"
 
 ## Make Helpers

scripts/update-packages

@@ -1,8 +1,15 @@
 #!/bin/bash
+[ -f /.dockerenv ] || { echo "please run in supplied container"; exit 1; }
 set -e
 
 apt update -y
-
 apt upgrade -y
 
 dpkg -l | awk '{ if ($1 == "ii") print $2 "=" $3 }' > /etc/apt/packages.list
+
+snapshot_date=$(date -d @${GIT_EPOCH} +"%Y%m%dT000000Z")
+cat <<-EOF > /etc/apt/sources.list
+deb http://snapshot.debian.org/archive/debian/${snapshot_date} buster main
+deb http://snapshot.debian.org/archive/debian-security/${snapshot_date} buster/updates main
+deb http://snapshot.debian.org/archive/debian/${snapshot_date} buster-updates main
+EOF