fix(docker-compose): fix test for `docker-compose` executable (#12555)

plugins/docker-compose/docker-compose.plugin.zsh

@@ -1,5 +1,8 @@
-# support Compose v2 as docker CLI plugin
-(( ${+commands[docker-compose]} )) && dccmd='docker-compose' || dccmd='docker compose'
+# Support Compose v2 as docker CLI plugin
+#
+# This tests that the (old) docker-compose command is in $PATH and that
+# it resolves to an existing executable file if it's a symlink.
+[[ -x "${commands[docker-compose]:A}" ]] && dccmd='docker-compose' || dccmd='docker compose'
 
 alias dco="$dccmd"
 alias dcb="$dccmd build"