adds distinct build stage to ci

also makes pandoc fail if there are any warnings, e.g. if images are
missing

.gitlab-ci.yml

 stages:
   - check
+  - build
   - deploy
 
 variables:
@@ -12,6 +13,13 @@ check:
   tags:
     - pandoc
 
+build:
+  stage: build
+  script:
+    - make
+  tags:
+    - pandoc
+
 deploy:
   stage: deploy
   script:

Makefile

@@ -3,6 +3,7 @@ TITLE = title
 PANDOC = pandoc
 
 PANDOCFLAGS = \
+	--fail-if-warnings \
 	--standalone \
 	--filter pandoc-citeproc \
 	--pdf-engine=xelatex \