ci: lint markdown files

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

ci/main.cue

@@ -7,9 +7,10 @@ import (
 	"universe.dagger.io/alpine"
 	"universe.dagger.io/docker"
 	"universe.dagger.io/go"
-	"universe.dagger.io/go/golangci"
 
+	"github.com/dagger/dagger/ci/pkg/golangci"
 	"github.com/dagger/dagger/ci/pkg/shellcheck"
+	"github.com/dagger/dagger/ci/pkg/markdownlint"
 )
 
 dagger.#Plan & {
@@ -96,6 +97,11 @@ dagger.#Plan & {
 				source: _source
 			}
 
+			markdown: markdownlint.#Lint & {
+				source: _source
+				files: ["./docs", "README.md"]
+			}
+
 			cue: docker.#Build & {
 				steps: [
 					alpine.#Build & {