Travis doctest docker (qgis#3618)

* Changes script to also run make doctest

Now, that we have some tests with Python doctest, we can/should make travis test them regularly.

I am not sure I know what doing here, so this is probably more a call for action than a proper PR.

* Use docker for doctest in Travis

.travis.yml

@@ -2,6 +2,9 @@ language: python
 python:
   - "3.5"
 
+services:
+  - docker
+
 cache:
   apt: true
   pip: true
@@ -50,6 +53,7 @@ install:
 
 script:
   - make html
+  - make -f docker.mk doctest
 
 #after_success:
 #  - ./scripts/ci/deploy-docs.sh

docker.mk

@@ -3,6 +3,7 @@
 doctest:
 	docker build -t qgis/documentation:latest -f doctest.dockerfile .
 	docker run --rm \
+		--env=QGIS_PREFIX_PATH=/usr \
 		--user=$(shell id -u) \
 		--volume=$(PWD):/documentation \
 		qgis/documentation:latest