Allow per os/python version Docker image selection

Author: altendky

.github/workflows/ci.yml

@@ -17,15 +17,20 @@ jobs:
   test:
     name: ${{ matrix.os.name }} ${{ matrix.python.name }} ${{ matrix.reactor.name }}
     runs-on: ${{ matrix.os.runs-on }}
-    container: ${{ format(matrix.os.container, matrix.python.docker) }}
+    container: ${{ matrix.os.container[matrix.python.docker] }}
     strategy:
       fail-fast: false
       matrix:
         os:
           - name: Linux
             runs-on: ubuntu-latest
             python_platform: linux
-            container: docker://python:{0}-buster
+            container:
+              2.7: docker://python:2.7-buster
+              3.5: docker://python:3.5-buster
+              3.6: docker://python:3.6-buster
+              3.7: docker://python:3.7-buster
+              3.8: docker://python:3.8-buster
           - name: Windows
             runs-on: windows-latest
             python_platform: win32