Add support for init on services

It's already supported by `swarmkit`, and act the same as
`HostConfig.Init` on container creation.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>
Upstream-commit: e401b88e59e098745744917c555d549f08353e6d
Component: engine

components/engine/api/swagger.yaml

@@ -2721,6 +2721,10 @@ definitions:
               - "default"
               - "process"
               - "hyperv"
+          Init:
+            description: "Run an init inside the container that forwards signals and reaps processes. This field is omitted if empty, and the default (as configured on the daemon) is used."
+            type: "boolean"
+            x-nullable: true
       NetworkAttachmentSpec:
         description: |
           Read-only spec type for non-swarm containers attached to swarm overlay

components/engine/api/types/swarm/container.go

@@ -55,6 +55,7 @@ type ContainerSpec struct {
 	User            string                  `json:",omitempty"`
 	Groups          []string                `json:",omitempty"`
 	Privileges      *Privileges             `json:",omitempty"`
+	Init            *bool                   `json:",omitempty"`
 	StopSignal      string                  `json:",omitempty"`
 	TTY             bool                    `json:",omitempty"`
 	OpenStdin       bool                    `json:",omitempty"`