Add shellcheck target, drop default

Makefile

@@ -1,5 +1,4 @@
-default: test
-.PHONY: test docopt kcov codecov
+.PHONY: test shellcheck docopt kcov codecov
 
 test:
 	@docker run \
@@ -9,6 +8,14 @@ test:
 		docker:dind \
 		apk add bats && bats test.bats
 
+shellcheck:
+	@docker run \
+		-it \
+		--rm \
+		-v $$(pwd):/workdir \
+		koalaman/shellcheck-alpine \
+		shellcheck /workdir/abra
+
 docopt:
 	@if [ ! -d ".venv" ]; then \
 		python3 -m venv .venv && \