[dependabot config] Remove invalid path (jaegertracing#5566)

## Which problem is this PR solving?
- The `/` path is invalid since there is no Dockerfile in the root dir.
Unfortunately, dependabot fails the whole run on this error, instead of
working through all the dirs
- Part of jaegertracing#5552

## Description of the changes
- Remove invalid path

## How was this change tested?
- Only testable after merge

Signed-off-by: Yuri Shkuro <github@ysh.us>

.github/dependabot.yml

@@ -33,7 +33,6 @@ updates:
 
   - package-ecosystem: docker
     directories:
-      - "/"
       - /cmd/agent
       - /cmd/all-in-one
       - /cmd/anonymizer