build-unit-test-docker: Add stdplus to the cache

This will soon be needed by sdeventplus and gpioplus.

Change-Id: Ia67b6a95ed33f61003721e5aa927b96fe88bc8e1
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 6591805..eec18d6 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -49,6 +49,7 @@
   phosphor-objmgr
   sdbusplus
   sdeventplus
+  stdplus
   gpioplus
   phosphor-logging
   phosphor-dbus-interfaces
@@ -277,6 +278,13 @@
 make -j$(nproc) && \
 make install
 
+FROM openbmc-base as openbmc-stdplus
+RUN curl -L https://github.com/openbmc/stdplus/archive/${PKG_REV['stdplus']}.tar.gz | tar -xz && \
+cd stdplus-* && \
+meson build -Dprefix=${PREFIX} -Dtests=disabled -Dexamples=false && \
+ninja -C build && \
+ninja -C build install
+
 FROM openbmc-base as openbmc-sdbusplus
 RUN curl -L https://github.com/openbmc/sdbusplus/archive/${PKG_REV['sdbusplus']}.tar.gz | tar -xz && \
 cd sdbusplus-* && \