Clarify the use of external Docker image

Author: JohnTitor

ci/docker/switch/Dockerfile

@@ -1,3 +1,6 @@
+# NOTE: the pacman that we use for this target doesn't support
+# to use it on CI and we should pull it from another Docker image.
+
 FROM huyuumi/libc-switch:latest
 
 RUN apt-get update && apt-get install -y --no-install-recommends \