Do not pre-pull cache images

Since 20.18.6 / 2140a2b / drone-plugins#462,
buildkit is always enabled.

Therefore, there is no need anymore to pre-pull the cached images,
`--cache-from` will cause buildkit to pull as needed.

This is a slimmed down alternative to drone-plugins#360.

Author: gzm0

docker.go

@@ -230,11 +230,6 @@ func (p Plugin) Exec() error {
 	cmds = append(cmds, commandVersion()) // docker version
 	cmds = append(cmds, commandInfo())    // docker info
 
-	// pre-pull cache images
-	for _, img := range p.Build.CacheFrom {
-		cmds = append(cmds, commandPull(img))
-	}
-
 	// setup for using ssh agent (https://docs.docker.com/develop/develop-images/build_enhancements/#using-ssh-to-access-private-data-in-builds)
 	if p.Build.SSHAgentKey != "" {
 		var sshErr error
@@ -363,10 +358,6 @@ func isCommandPull(args []string) bool {
 	return len(args) > 2 && args[1] == "pull"
 }
 
-func commandPull(repo string) *exec.Cmd {
-	return exec.Command(dockerExe, "pull", repo)
-}
-
 func commandLoginEmail(login Login) *exec.Cmd {
 	return exec.Command(
 		dockerExe, "login",