Remove docker image definition

It's not built, nor published, nor used by any releng task as explained
in eclipse-platform/eclipse.platform.releng.aggregator#2401

Author: akurtakov

.github/dependabot.yml

@@ -5,11 +5,6 @@ updates:
     schedule:
       interval: daily
 
-  - package-ecosystem: docker
-    directory: /container
-    schedule:
-      interval: daily
-
   - package-ecosystem: maven
     directory: /
     schedule: