docs(get-images-from-files): document compose support (#28)

get-images-from-files already supports compose files. This PR explicitly documents the compatibility and includes a test.

Reviewed-on: t.behrendt/trivy-actions#28
Reviewed-by: branch-buddy <branch-buddy@t00n.de>
Co-authored-by: t.behrendt <t.behrendt@t00n.de>
Co-committed-by: t.behrendt <t.behrendt@t00n.de>

get-images-from-files/action.yaml

@@ -1,5 +1,5 @@
 name: "Get Images From Files"
-description: "Extract docker images from selected files (Dockerfiles, Kubernetes manifests, and files with recognizable image references)"
+description: "Extract docker images from selected files (Dockerfiles, Kubernetes manifests, Docker Compose/compose files, and files with recognizable image references)"
 author: "Timo Behrendt <t.behrendt@t00n.de>"
 branding:
   icon: "package"
@@ -30,7 +30,7 @@ runs:
             [ -f "$f" ] || continue
             # Dockerfile: FROM [--platform=...] <image>
             awk '/^[[:space:]]*FROM[[:space:]]/ { print $NF }' "$f" 2>/dev/null || true
-            # Kubernetes / Compose: image: <ref>
+            # Kubernetes / Docker Compose: image: <ref>
             grep -E '[[:space:]]image:[[:space:]]' "$f" 2>/dev/null | sed 's/.*image:[[:space:]]*//' | awk '{ print $1 }' || true
           done | grep -E '/' | sort -u | paste -sd,
         )