virt: rework container detection logic

Instead of accessing /proc/1/environ directly, trying to read the
$container variable from it, let's make PID 1 save the contents of that
variable to /run/systemd/container. This allows us to detect containers
without the need for CAP_SYS_PTRACE, which allows us to drop it from a
number of daemons and from the file capabilities of systemd-detect-virt.

Also, don't consider chroot a container technology anymore. After all,
we don't consider file system namespaces container technology anymore,
and hence chroot() should be considered a container even less.

src/core/main.c

@@ -1261,6 +1261,16 @@ static int status_welcome(void) {
                              isempty(pretty_name) ? "Linux" : pretty_name);
 }
 
+static int write_container_id(void) {
+        const char *c;
+
+        c = getenv("container");
+        if (isempty(c))
+                return 0;
+
+        return write_string_file("/run/systemd/container", c);
+}
+
 int main(int argc, char *argv[]) {
         Manager *m = NULL;
         int r, retval = EXIT_FAILURE;
@@ -1544,6 +1554,8 @@ int main(int argc, char *argv[]) {
                 if (virtualization)
                         log_info("Detected virtualization '%s'.", virtualization);
 
+                write_container_id();
+
                 log_info("Detected architecture '%s'.", architecture_to_string(uname_architecture()));
 
                 if (in_initrd())