Improve the init system: support skipping startup files and running a custom main command

README.md

@@ -40,6 +40,7 @@ You can configure the stock `ubuntu` image yourself from your Dockerfile, so why
    * [Getting started](#getting_started)
    * [Adding additional daemons](#adding_additional_daemons)
    * [Running scripts during container startup](#running_startup_scripts)
+   * [Running a one-shot command in the container](#oneshot)
    * [Login to the container via SSH](#login)
  * [Building the image yourself](#building)
  * [Conclusion](#conclusion)
@@ -154,6 +155,50 @@ The following example shows how you can add a startup script. This script simply
     RUN mkdir -p /etc/my_init.d
     ADD logtime.sh /etc/my_init.d/logtime.sh
 
+<a name="oneshot"></a>
+### Running a one-shot command in the container
+
+Normally, when you want to run a single command in a container, and exit immediately after the command, you invoke Docker like this:
+
+    docker run YOUR_IMAGE COMMAND ARGUMENTS...
+
+However the downside of this approach is that the init system is not started. That is, while invoking `COMMAND`, important daemons such as cron and syslog are not running. Also, orphaned child processes are not properly reaped, because `COMMAND` is PID 1.
+
+Baseimage-docker provides a facility to run a single one-shot command, while solving all of the aforementioned problems. Run a single command in the following manner:
+
+    docker run YOUR_IMAGE /sbin/my_init -- COMMAND ARGUMENTS ...
+
+This will perform the following:
+
+ * Runs all system startup files, such as /etc/my_init.d/* and /etc/rc.local.
+ * Starts all runit services.
+ * Runs the specified command.
+ * When the specified command exits, stops all runit services.
+
+For example:
+
+    $ docker run phusion/baseimage:<VERSION> /sbin/my_init -- ls
+    *** Running /etc/my_init.d/00_regen_ssh_host_keys.sh...
+    No SSH host key available. Generating one...
+    Creating SSH2 RSA key; this may take some time ...
+    Creating SSH2 DSA key; this may take some time ...
+    Creating SSH2 ECDSA key; this may take some time ...
+    *** Running /etc/rc.local...
+    *** Booting runit daemon...
+    *** Runit started as PID 80
+    *** Running ls...
+    bin  boot  dev  etc  home  image  lib  lib64  media  mnt  opt  proc  root  run  sbin  selinux  srv  sys  tmp  usr  var
+    *** ls exited with exit code 0.
+    *** Shutting down runit daemon (PID 80)...
+    *** Killing all processes...
+
+You may find that the default invocation is too noisy. Or perhaps you don't want to run the startup files. You can customize all this by passing arguments to `my_init`. Invoke `docker run YOUR_IMAGE /sbin/my_init --help` for more information.
+
+The following example runs `ls` without running the startup files and with less messages, while running all runit services:
+
+    $ docker run phusion/baseimage:<VERSION> /sbin/my_init --skip-startup-files --quiet -- ls
+    bin  boot  dev  etc  home  image  lib  lib64  media  mnt  opt  proc  root  run  sbin  selinux  srv  sys  tmp  usr  var
+
 <a name="login"></a>
 ### Login to the container via SSH