tast: Add "containers" dependency.

This is needed to skip the security.RunOCI test on DUTs that
don't have run_oci installed.

BUG=chromium:877733
TEST=none
CQ-DEPEND=Icb89c58f14ef4e82734faa77381d063ddbd1741b

Change-Id: I15dd377241628b51c7a182357bd676b5bbf2ee60
Reviewed-on: https://chromium-review.googlesource.com/1382621
Commit-Ready: Dan Erat <[email protected]>
Tested-by: Dan Erat <[email protected]>
Reviewed-by: Shuhei Takahashi <[email protected]>
Reviewed-by: Hidehiko Abe <[email protected]>
diff --git a/docs/test_dependencies.md b/docs/test_dependencies.md
index 2a21a48..848a675 100644
--- a/docs/test_dependencies.md
+++ b/docs/test_dependencies.md
@@ -24,6 +24,8 @@
     rather than Chromium. (e.g. proprietary codec like H.264)
 *   `chrome_login` - Implies `chrome` with the further requirement that user
     login (i.e. using `session_manager` and `cryptohome`) is supported.
+*   `containers` - The ability to run code within containers. This implies the
+    presence of the `run_oci` program.
 *   `cups` - CUPS daemon.
 *   `display_backlight` - An internal display backlight.
 *   `gpu_sandboxing` - Chrome's GPU process is [sandboxed].