build-unit-test-docker: sdeventplus will now depend on stdplus

This is now a hard requirement for building sdeventplus

Change-Id: I9014b9350b6e01d627f35a0cd02d3ca14e3cb82e
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 eec18d6..fc764f4 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -294,6 +294,7 @@
 make install
 
 FROM openbmc-base as openbmc-sdeventplus
+COPY --from=openbmc-stdplus ${PREFIX} ${PREFIX}
 RUN curl -L https://github.com/openbmc/sdeventplus/archive/${PKG_REV['sdeventplus']}.tar.gz | tar -xz && \
 cd sdeventplus-* && \
 meson build -Dprefix=${PREFIX} -Dtests=disabled -Dexamples=false && \