Document the main Makefile a little less poorly

Makefile

@@ -3,18 +3,32 @@ define INC_DOCKER =
 endef
 $(info Defined INC_DOCKER)
 
-include /devel/docker/docker_vars.sh
+docker_src = /devel/docker
+include ${docker_src}/docker_vars.sh
 export
 
+# A container is built based on a given Dockerfile.
+# We mark a sentinel when a container is built.
 ${docker_src}/built/%.sentinel: ${docker_src}/%.dockerfile
 	grep -e '^FROM' $< | awk '{print $$2}' \
 		| tr : _ | xargs -I{} $(MAKE) ${docker_src}/built/{}.sentinel;
 	cd ${docker_src}; docker build -f $< --tag $* .;
 	touch $@;
 
+# Exceptions to this rule are listed below, but should handle the rule
+# the same ways.
 ${docker_src}/built/debian_10.sentinel:
 	docker pull "debian:10" && touch $@;
 
+# For example, the main openwrt builder container is built specific to
+# a remote and a commit. [Basing this container on a main
+# openwrt-builder container still saves us time as OpenWrt reomtes
+# reliably share almost all of the same history.]
+##
+# Two formats are accepted: openwrt-builder-${rev}.sentinel for the
+# base builder on the default remote at a particular revision, and
+# openwrt-builder-${remote}_${rev}.sentinel to construct the builder
+# pointing to a particular remote at a particular revision.
 ${docker_src}/built/openwrt-builder-%.sentinel: \
  ${docker_src}/openwrt-builder-commit.dockerfile \
  ${docker_src}/built/openwrt-builder.sentinel \