Add post-start hooks

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

runtime-config.md

@@ -6,6 +6,7 @@ Lifecycle hooks allow custom events for different points in a container's runtim
 Presently there are `Prestart` and `Poststop`.
 
 * [`Prestart`](#pre-start) is a list of hooks to be run before the container process is executed
+* [`Poststart`](#post-start) is a list of hooks to be run immediately after process in container started
 * [`Poststop`](#post-stop)is a list of hooks to be run after the container process exits
 
 Hooks allow one to run code before/after various lifecycle events of the container.
@@ -22,6 +23,13 @@ In Linux, for e.g., the network namespace could be configured in this hook.
 
 If a hook returns a non-zero exit code, then an error including the exit code and the stderr is returned to the caller and the container is torn down.
 
+### Post-start
+
+The post-start hooks are called after the user process is started.
+It can be used for notifying user that real process is spawned.
+
+If a hook returns a non-zero exit code, then an error is logged and the remaining hooks are executed.
+
 ### Post-stop
 
 The post-stop hooks are called after the container process is stopped.
@@ -42,6 +50,11 @@ If a hook returns a non-zero exit code, then an error is logged and the remainin
                 "path": "/usr/bin/setup-network"
             }
         ],
+        "poststart": [
+            {
+                "path": "/usr/bin/notify-start",
+            },
+        ],
         "poststop": [
             {
                 "path": "/usr/sbin/cleanup.sh",

runtime_config.go

@@ -22,6 +22,8 @@ type Hooks struct {
 	// Prestart is a list of hooks to be run before the container process is executed.
 	// On Linux, they are run after the container namespaces are created.
 	Prestart []Hook `json:"prestart"`
+	// Poststart is a list of hooks to be run after the container process is started.
+	Poststart []Hook `json:"poststart"`
 	// Poststop is a list of hooks to be run after the container process exits.
 	Poststop []Hook `json:"poststop"`
 }