Don't test heavily outdated Docker versions anymore

.github/workflows/it.yml

@@ -69,18 +69,6 @@ jobs:
           runs-on: ubuntu-20.04
         - dind-image: docker:18.06-dind
           runs-on: ubuntu-20.04
-        - dind-image: docker:18.03-dind
-          runs-on: ubuntu-20.04
-        - dind-image: docker:17.12-dind
-          runs-on: ubuntu-20.04
-        - dind-image: docker:17.09-dind
-          runs-on: ubuntu-20.04
-        - dind-image: docker:17.07-dind
-          runs-on: ubuntu-20.04
-        - dind-image: docker:17.06-dind
-          runs-on: ubuntu-20.04
-        - dind-image: docker:1.13-dind
-          runs-on: ubuntu-20.04
 
     services:
       dind:

README.md

@@ -339,14 +339,8 @@ DFW is continuously and automatically tested with the following stable Docker ve
 * `19.03`
 * `18.09`
 * `18.06`
-* `18.03`
-* `17.12`
-* `17.09`
-* `17.07`
-* `17.06`
-* `1.13`
-
-Docker version 20.10 is also officially supported by DFW, although it is not yet automatically tested.
+
+Docker versions between 18.03 and 1.13 should also work, but are not automatically tested anymore due to lacking Docker BuildKit support.
 
 ## <a name="versionbumppolicy"></a> Version bump policy