make cache-from an array

docker.go

@@ -47,7 +47,7 @@ type (
 		Target      string   // Docker build target
 		Squash      bool     // Docker build squash
 		Pull        bool     // Docker build pull
-		CacheFrom   string   // Docker build cache-from
+		CacheFrom   []string // Docker build cache-from
 		Compress    bool     // Docker build compress
 		Repo        string   // Docker build repository
 		LabelSchema []string // label-schema Label map
@@ -106,8 +106,8 @@ func (p Plugin) Exec() error {
 	}
 
 	// pre-pull cache image
-	if p.Build.CacheFrom != "" {
-		cmd := commandPull(p.Build.CacheFrom)
+	for _, img := range p.Build.CacheFrom {
+		cmd := commandPull(img)
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
 		trace(cmd)
@@ -218,8 +218,8 @@ func commandBuild(build Build) *exec.Cmd {
 	if build.NoCache {
 		args = append(args, "--no-cache")
 	}
-	if build.CacheFrom != "" {
-		args = append(args, "--cache-from", build.CacheFrom)
+	for _, arg := range build.CacheFrom {
+		args = append(args, "--cache-from", arg)
 	}
 	for _, arg := range build.ArgsEnv {
 		addProxyValue(&build, arg)