build-unit-test-docker: Fix linter errors

It's best practice to quote variable substitutions inteded to not span
multiple command line arguments anyway.

Change-Id: I92ecd3ddc42881dd2e7dd3248eeedafeb7cfe6ab
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 efd4d7d..0caae48 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -401,7 +401,7 @@
 
 # Final configuration for the workspace
 RUN grep -q ${GROUPS} /etc/group || groupadd -g ${GROUPS} ${USER}
-RUN mkdir -p $(dirname ${HOME})
+RUN mkdir -p "$(dirname "${HOME}")"
 RUN grep -q ${UID} /etc/passwd || useradd -d ${HOME} -m -u ${UID} -g ${GROUPS} ${USER}
 RUN sed -i '1iDefaults umask=000' /etc/sudoers
 RUN echo "${USER} ALL=(ALL) NOPASSWD: ALL" >>/etc/sudoers