Add detection for docker containers

etc/rc.conf.Linux

@@ -3,6 +3,7 @@
 
 # This is the subsystem type. Valid options on Linux:
 # ""               - nothing special
+# "docker"         - Docker container manager
 # "lxc"            - Linux Containers
 # "openvz"         - Linux OpenVZ
 # "prefix"         - Prefix

man/openrc-run.8

@@ -223,6 +223,8 @@ Same as -jail, but for VServer systems.
 Same as -jail, but for Xen DOM0 systems.
 .It Dv -xenu
 Same as -jail, but for Xen DOMU systems.
+.It Dv -docker
+Same as -jail, but for docker systems.
 .El
 .El
 .Pp

src/librc/librc.c

@@ -269,6 +269,8 @@ rc_sys(void)
 			return RC_SYS_LXC;
 		if (strcmp(systype, RC_SYS_SYSTEMD_NSPAWN) == 0)
 				return RC_SYS_SYSTEMD_NSPAWN;
+		if (strcmp(systype, RC_SYS_DOCKER) == 0)
+				return RC_SYS_DOCKER;
 	}
 	if (exists("/proc/xen")) {
 		if (file_regex("/proc/xen/capabilities", "control_d"))
@@ -288,6 +290,8 @@ rc_sys(void)
 		return RC_SYS_LXC;
 	else if (file_regex("/proc/1/environ", "container=systemd-nspawn"))
 		return RC_SYS_SYSTEMD_NSPAWN;
+	else if (file_regex("/proc/1/environ", "container=docker"))
+		return RC_SYS_DOCKER;
 #endif
 
 	return NULL;

src/librc/rc.h.in

@@ -329,6 +329,7 @@ bool rc_service_daemons_crashed(const char *);
 /*! @name System types
  * OpenRC can support some special sub system types, normally virtualization.
  * Some services cannot work in these systems, or we do something else. */
+#define RC_SYS_DOCKER  "DOCKER"
 #define RC_SYS_JAIL    "JAIL"
 #define RC_SYS_OPENVZ  "OPENVZ"
 #define RC_SYS_LXC     "LXC"