feature: add post mount commands step in chroot builder

builder/osc/chroot/builder.go

@@ -256,6 +256,9 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 			MountOptions:   b.config.MountOptions,
 			MountPartition: b.config.MountPartition,
 		},
+		&StepPostMountCommands{
+			Commands: b.config.PostMountCommands,
+		},
 	)
 
 	// Run!

builder/osc/chroot/step_post_mount_commands.go

@@ -0,0 +1,47 @@
+package chroot
+
+import (
+	"context"
+
+	"github.com/hashicorp/packer/helper/multistep"
+	"github.com/hashicorp/packer/packer"
+)
+
+type postMountCommandsData struct {
+	Device    string
+	MountPath string
+}
+
+// StepPostMountCommands allows running arbitrary commands after mounting the
+// device, but prior to the bind mount and copy steps.
+type StepPostMountCommands struct {
+	Commands []string
+}
+
+func (s *StepPostMountCommands) Run(_ context.Context, state multistep.StateBag) multistep.StepAction {
+	config := state.Get("config").(*Config)
+	device := state.Get("device").(string)
+	mountPath := state.Get("mount_path").(string)
+	ui := state.Get("ui").(packer.Ui)
+	wrappedCommand := state.Get("wrappedCommand").(CommandWrapper)
+
+	if len(s.Commands) == 0 {
+		return multistep.ActionContinue
+	}
+
+	ctx := config.ctx
+	ctx.Data = &postMountCommandsData{
+		Device:    device,
+		MountPath: mountPath,
+	}
+
+	ui.Say("Running post-mount commands...")
+	if err := RunLocalCommands(s.Commands, wrappedCommand, ctx, ui); err != nil {
+		state.Put("error", err)
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+	return multistep.ActionContinue
+}
+
+func (s *StepPostMountCommands) Cleanup(state multistep.StateBag) {}