kokoro: Fix docker build step in simulator script

The old command no longer works as of CL:1419397 because the pkgconfig
path in COPY needs to be interpreted relative to the Dockerfile path.
The kokoro/README.md instructions were updated in CL:1431356 but the
simulator script was not.

Patch received via email from Gabriel <g2p.code@gmail.com>.
0e6bb00b90

TEST=run kokoro simulator

Change-Id: I09d556cab9a4c1942098e0796e39e221f365c3af
Reviewed-on: https://chromium-review.googlesource.com/1513497
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: David Tolnay <dtolnay@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Stephen Barber <smbarber@chromium.org>

kokoro/kokoro_simulator.sh

@@ -16,7 +16,7 @@ main() {
   mkdir -p "${kokoro_simulator_root}"
   if [[ ! -e "${base_image_tarball}" ]]; then
     if [[ "$(docker images -q ${base_image} 2> /dev/null)" == "" ]]; then
-      docker build -t ${base_image} - < Dockerfile
+      docker build -t ${base_image} .
     fi
     docker save ${base_image} | xz -T 0 -z >"${base_image_tarball}"
   fi