Add Nlohmann-json to build phosphor-ipmi-host

The nlohmann package was recently added to the sysroot in Yocto.
Need to add it here in order to build phosphor-ipmi-host out of tree.

Change-Id: I52fc2b113e52e4497417509b581b6edac5f8fa6b
Signed-off-by: David Cobbley <david.j.cobbley@linux.intel.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 37c8f28..7c9249a 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -89,6 +89,10 @@
 RUN tar -xzf v1.2.2.tar.gz
 RUN cp -a cereal-1.2.2/include/cereal/ /usr/include/
 
+RUN wget https://github.com/nlohmann/json/releases/download/v3.0.0/json.hpp
+RUN mkdir /usr/include/nlohmann-json/
+RUN cp -a json.hpp /usr/include/nlohmann-json/
+
 RUN grep -q ${GROUPS} /etc/group || groupadd -g ${GROUPS} ${USER}
 RUN grep -q ${UID} /etc/passwd || useradd -d ${HOME} -m -u ${UID} -g ${GROUPS} ${USER}