Makefile: don't warn "outside container" for some targets

This change allows some make targets to be ran outside the dev-container for
easier discovery and use:

- `make clean` can be used on the host (as artifacts created from within the
  development container are usually stored on the host).
- `make help` was already allowed
- `make dev` and `make shell` are added to the regular Makefile, to make it
  easier to create and start the development container.
- When attempting to run `make dev` from within the development container, a
  message is printed, and the target is cancelled:

      root@docker-cli-dev$ make dev
      you are already in the dev container

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Makefile

@@ -9,6 +9,17 @@ all: binary
 
 _:=$(shell ./scripts/warn-outside-container $(MAKECMDGOALS))
 
+.PHONY: dev
+dev: ## start a build container in interactive mode for in-container development
+	@if [ -n "${DISABLE_WARN_OUTSIDE_CONTAINER}" ]; then \
+		echo "you are already in the dev container"; \
+	else \
+		$(MAKE) -f docker.Makefile dev; \
+	fi
+
+.PHONY: shell
+shell: dev ## alias for dev
+
 .PHONY: clean
 clean: ## remove build artifacts
 	rm -rf ./build/* man/man[1-9] docs/yaml

scripts/warn-outside-container

@@ -3,16 +3,25 @@ set -eu
 
 target="${1:-}"
 
-if [ "$target" != "help" ] && [ -z "${DISABLE_WARN_OUTSIDE_CONTAINER:-}" ]; then
+if [ -z "${DISABLE_WARN_OUTSIDE_CONTAINER:-}" ]; then
+		case $target in
+			clean|dev|help|shell)
+				# no warning needed for these targets
+				;;
+			*)
 				(
 						echo
+						echo "\033[1mWARNING\033[0m: you are not in a container."
 						echo
-        echo "WARNING: you are not in a container."
-        echo "Use \"make -f docker.Makefile $target\" or set"
-        echo "DISABLE_WARN_OUTSIDE_CONTAINER=1 to disable this warning."
+						echo 'Use "\033[1mmake dev\033[0m" to start an interactive development container,'
+						echo "use \"\033[1mmake -f docker.Makefile $target\033[0m\" to execute this target"
+						echo "in a container, or set \033[1mDISABLE_WARN_OUTSIDE_CONTAINER=1\033[0m to"
+						echo "disable this warning."
 						echo
-        echo "Press Ctrl+C now to abort."
+						echo "Press \033[1mCtrl+C\033[0m now to abort, or wait for the script to continue.."
 						echo
 				) >&2
-    sleep 10
+				sleep 5
+				;;
+		esac
 fi