- What I did
Add stopsignal in validCommitCommands for commit command.

- How I did it

• docker commit command requests CreateImageFromContainer function.

  moby/daemon/commit.go

  Line 119 in a583434

  func (daemon *Daemon) CreateImageFromContainer(name string, c *backend.CreateImageConfig) (string, error) {

• This function calls dockerfile.BuildFromConfig function
  

  moby/daemon/commit.go

  Line 149 in a583434

  newConfig, err := dockerfile.BuildFromConfig(c.Config, c.Changes, container.OS)

• validCommitCommands is used to rejected unsupported instructions in dockerfile.BuildFromConfig
  

  moby/builder/dockerfile/builder.go

  Line 339 in a583434

  if !validCommitCommands[n.Value] {

• Back to CreateImageFromContainer function, merge function already support StopSIgnal
  

  moby/daemon/commit.go

  Line 110 in a583434

  if userConf.StopSignal == "" {

- How to verify it
Commit a new image with following command: docker commit xxxx -c "STOPSIGNAL SIGRTMIN+3"

- Description for the changelog

Allow STOPSIGNAL instruction in commit changes

- A picture of a cute animal (not mandatory but encouraged)