Move to Ruby 2.7 for the default Docker version as well as the default target for alternative DB's. Fix the Ruby image version with three digits because the most recent versions seem to be somehow broken.

test-envs/docker-compose-2.6-mysql.yml

@@ -10,7 +10,7 @@ services:
   web:
     build:
       context: ..
-      dockerfile: Dockerfile
+      dockerfile: Dockerfile-2.6
     environment:
       # These are set in script/ci-build, so we need to pass-thru them.
       RAILS_ENV: $RAILS_ENV

test-envs/docker-compose-2.7-mysql.yml

@@ -10,7 +10,7 @@ services:
   web:
     build:
       context: ..
-      dockerfile: Dockerfile-2.7
+      dockerfile: Dockerfile
     environment:
       # These are set in script/ci-build, so we need to pass-thru them.
       RAILS_ENV: $RAILS_ENV