feat(platforms): fall back to ubuntu-latest image for ubuntu-slim runner

cmd/platforms.go

@@ -7,6 +7,7 @@ import (
 func (i *Input) newPlatforms() map[string]string {
 	platforms := map[string]string{
 		"ubuntu-latest": "node:16-buster-slim",
+		"ubuntu-slim":   "node:16-buster-slim",
 		"ubuntu-22.04":  "node:16-bullseye-slim",
 		"ubuntu-20.04":  "node:16-buster-slim",
 		"ubuntu-18.04":  "node:16-buster-slim",

cmd/platforms_test.go

@@ -0,0 +1,31 @@
+package cmd
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewPlatformsDefaults(t *testing.T) {
+	i := &Input{}
+	platforms := i.newPlatforms()
+
+	// ubuntu-latest must be present and resolve to a default image
+	latest, ok := platforms["ubuntu-latest"]
+	assert.True(t, ok, "ubuntu-latest should be a default platform")
+	assert.NotEmpty(t, latest)
+
+	// ubuntu-slim should fall back to the same image as ubuntu-latest
+	// (GitHub's 1-vCPU runner uses the same base distro, see issue #6003)
+	slim, ok := platforms["ubuntu-slim"]
+	assert.True(t, ok, "ubuntu-slim should be a default platform")
+	assert.Equal(t, latest, slim, "ubuntu-slim should default to the same image as ubuntu-latest")
+}
+
+func TestNewPlatformsUserOverride(t *testing.T) {
+	i := &Input{platforms: []string{"ubuntu-slim=catthehacker/ubuntu:act-latest"}}
+	platforms := i.newPlatforms()
+
+	assert.Equal(t, "catthehacker/ubuntu:act-latest", platforms["ubuntu-slim"],
+		"user-supplied -P value should override the default")
+}