docker: add chronos user to docker group

enable sudo-less docker for the chronos user.

BUG=b:272072684
TEST=run "docker run hello-world:latest"

Change-Id: I04f31eba15616ac190615248fb9ccf25492fb681
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/4363459
Commit-Queue: Brett Brotherton <bbrotherton@google.com>
Tested-by: Parker Holloway <jparkerh@google.com>
Tested-by: Brett Brotherton <bbrotherton@google.com>
Reviewed-by: Brett Brotherton <bbrotherton@google.com>
Reviewed-by: Allen Webb <allenwebb@google.com>
diff --git a/profiles/base/accounts/group/docker b/profiles/base/accounts/group/docker
index 7816a8b..91f72a7 100644
--- a/profiles/base/accounts/group/docker
+++ b/profiles/base/accounts/group/docker
@@ -1,3 +1,4 @@
 group:docker
 gid:412
 password:!
+users:chronos