Add npm

npm will be used in the phosphor-webui to install eslint which
will be used  to lint JavaScript files.
npm will also be used to install prettier which will be used by
phosphor-webui to style JavaScript and scss files.

Change-Id: I241f3c3c5b16e8f48a3a8dba3b9294221bf14220
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 997eee3..74af1df 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -204,6 +204,7 @@
     clang-format-8 \
     clang-tidy-8 \
     clang-tools-8 \
+    npm \
     iproute2 \
     libnl-3-dev \
     libnl-genl-3-dev \