shellcheck: enable shell linting

Add to the `format-code.sh` optional support for linting shell scripts
using `shellcheck`.  All repositories will have this ran, but only
repositories with a `.shellcheck` file in the root of the repository
will stop on error.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ie25c27cffc96b7fa66291abef1b0f90a0f1862ed
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index d10857f..bea8030 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -239,6 +239,7 @@
     clang-format-10 \
     clang-tidy-10 \
     clang-tools-10 \
+    shellcheck \
     npm \
     iproute2 \
     libnl-3-dev \