run Travis tests in CentOS container

.travis.yml

-language: python
+language: minimal
 env:
-    - PYTHON_VER=2.7
-    - PYTHON_VER=3.4
     - PYTHON_VER=3.5
     - PYTHON_VER=3.6
+    - PYTHON_VER=3.7
+    - PYTHON_VER=3.8
 
-sudo: false
-cache:
-  directories:
-  - pythons
-# Note on this cache directories: "make build" downloads pythons elsewhere,
-# but installs the compiled bin/lib/share in pythons/pythonx.y/ .
+services:
+    - docker
 
 notifications:
   email:
     - buildout-development@googlegroups.com
 
 install:
-    - ls -al pythons
-    - export COVERAGE_PROCESS_START=
-    - deactivate
-    - make build
+    - docker build -f .github/workflows/Dockerfile --tag centos_buildout:python${PYTHON_VER} --build-arg PYTHON_VER=${PYTHON_VER} .
 
-script: make test
+script:
+    - docker run centos_buildout:python${PYTHON_VER} /buildout/bin/test -c -vvv