Add sphinx linkcheck to travis build

.travis.yml

@@ -7,5 +7,5 @@ services:
 install:
     - make dev-env
 script:
-    - make docs
+    - make test/docs docs
     - make build-test-all DARGS="--build-arg TEST_ONLY_BUILD=1"

Makefile

@@ -60,6 +60,9 @@ dev-env: ## install libraries required to build docs and run tests
 docs: ## build HTML documentation
 	make -C docs html
 
+test/docs: ## check links in Sphinx documentation
+	make -C docs linkcheck
+
 test/%: ## run tests against a stack
 	@TEST_IMAGE="$(OWNER)/$(notdir $@)" pytest test