build-unit-test-docker: Workaround COPY bug

This patch adds a workaround in an attempt to fix random breakage during
CI runs that looks like:

11:17:07 Step 64/71 : COPY --from=openbmc-sdeventplus /usr/local /usr/local
11:17:12 failed to export image: failed to create image: failed to get layer sha256:01b7c3b6a1d0dda1564d6d061ca67c52939c21a39e03257dda8aa9daf53d2d50: layer does not exist.

It was noted in https://github.com/moby/moby/issues/37965 that adding
non-COPY operations in between the COPYs could work around these
failures.

Change-Id: Ie0e8e92988a95dca1fcf9a317bbf4091d77e061f
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index cbbab1c..d864208 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -118,6 +118,9 @@
 # docker file and rebuild the image unnecessarily
 for pkg in $(echo "${!PKG_REV[@]}" | tr ' ' '\n' | LC_COLLATE=C sort -s); do
   COPY_CMDS+="COPY --from=openbmc-${pkg} ${PREFIX} ${PREFIX}"$'\n'
+  # Workaround for upstream docker bug and multiple COPY cmds
+  # https://github.com/moby/moby/issues/37965
+  COPY_CMDS+="RUN true"$'\n'
 done
 
 ################################# docker img # #################################