Use a specific docker tag image version

.travis.yml

@@ -18,17 +18,17 @@ matrix:
     - python: '3.5'
     - python: '3.4'
     - python: '3.7-dev'
-    - env: DOCKER="alpine"
-    - env: DOCKER="arch" # contains PyQt5
-    - env: DOCKER="ubuntu-trusty-x86"
-    - env: DOCKER="ubuntu-xenial-amd64"
-    - env: DOCKER="debian-stretch-x86"
-    - env: DOCKER="centos-6-amd64"
-    - env: DOCKER="centos-7-amd64"
-    - env: DOCKER="amazon-1-amd64"
-    - env: DOCKER="amazon-2-amd64"
-    - env: DOCKER="fedora-26-amd64"
-    - env: DOCKER="fedora-27-amd64"
+    - env: DOCKER="alpine" DOCKER_TAG="pytest"
+    - env: DOCKER="arch" DOCKER_TAG="pytest" # contains PyQt5
+    - env: DOCKER="ubuntu-trusty-x86" DOCKER_TAG="pytest"
+    - env: DOCKER="ubuntu-xenial-amd64" DOCKER_TAG="pytest"
+    - env: DOCKER="debian-stretch-x86" DOCKER_TAG="pytest"
+    - env: DOCKER="centos-6-amd64" DOCKER_TAG="pytest"
+    - env: DOCKER="centos-7-amd64" DOCKER_TAG="pytest"
+    - env: DOCKER="amazon-1-amd64" DOCKER_TAG="pytest"
+    - env: DOCKER="amazon-2-amd64" DOCKER_TAG="pytest"
+    - env: DOCKER="fedora-26-amd64" DOCKER_TAG="pytest"
+    - env: DOCKER="fedora-27-amd64" DOCKER_TAG="pytest"
 
 dist: trusty
 
@@ -41,7 +41,7 @@ install:
   - if [ "$DOCKER" == "" ]; then .travis/install.sh; fi
 
 before_install:
-  - if [ "$DOCKER" ]; then docker pull pythonpillow/$DOCKER; fi
+  - if [ "$DOCKER" ]; then docker pull pythonpillow/$DOCKER:$DOCKER_TAG; fi
 
 before_script:
 # Qt needs a display for some of the tests, and it's only run on the system site packages install
@@ -55,7 +55,7 @@ script:
        else
           # the Pillow user in the docker container is UID 1000
           sudo chown -R 1000 $TRAVIS_BUILD_DIR
-          docker run -v $TRAVIS_BUILD_DIR:/Pillow pythonpillow/$DOCKER
+          docker run -v $TRAVIS_BUILD_DIR:/Pillow pythonpillow/$DOCKER:$DOCKER_TAG
        fi
 
 after_success: