format-code: enable markdownlint

Enable markdownlint but allow it to always pass.  This will allow
maintainers the opportunity to run the tools and fix up their own
markdown before we enable this globally.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ie397a72ca4ef113cdbe433d263ccf3bd5063cf4b
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index c7bb94a..c52fad4 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -859,6 +859,7 @@
 
 RUN npm install -g \
         eslint@latest eslint-plugin-json@latest \
+        markdownlint-cli@latest \
         prettier@latest
 """