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