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 & {

ci/pkg/markdownlint/markdownlint.cue

@@ -0,0 +1,36 @@
+package markdownlint
+
+import (
+	"dagger.io/dagger"
+
+	"universe.dagger.io/docker"
+)
+
+#Lint: {
+	// Source code
+	source: dagger.#FS
+
+	// shellcheck version
+	version: *"0.31.1" | string
+
+	// Files to lint
+	files: [...string]
+
+	_image: docker.#Pull & {
+		source: "tmknom/markdownlint:\(version)"
+	}
+
+	container: docker.#Run & {
+		input: _image.output
+		mounts: "source": {
+			dest:     "/src"
+			contents: source
+		}
+		workdir: "/src"
+		command: {
+			// FIXME: this should not be required
+			name: "markdownlint"
+			args: files
+		}
+	}
+}