Fix *-debug docker images to be ":debug" tag based

We use the gcr.io/distroless/base:debug tag for the *-debug images of
OPA. At the point where we switched to the multi-stage build we dropped
defining the `VARIANT` argument so it wasn't being picked up correctly.

This lead to us just using gcr.io/distroless/base for the *-debug
image.. which isn't what we wanted.

Signed-off-by: Patrick East <east.patrick@gmail.com>

Dockerfile

@@ -1,6 +1,7 @@
 # Copyright 2019 The OPA Authors.  All rights reserved.
 # Use of this source code is governed by an Apache2
 # license that can be found in the LICENSE file.
+ARG VARIANT
 ARG BUILD_COMMIT
 # we cant use build-args in `COPY --from=...` below, so work around this
 # see: https://medium.com/@tonistiigi/advanced-multi-stage-build-patterns-6f741b852fae