Retire the docker ecosystem from Dependabot

The base image tag is a build arg — `ARG RUBY_VERSION` plus
`FROM ruby:$RUBY_VERSION-slim` — and Dependabot's docker updater matches
literal tags, so this entry has never had anything to propose. It ran
green every week and reported nothing, which reads as coverage and
isn't.

No other repo in the fleet configures a docker ecosystem, and none
could: they all either interpolate a variable or pull from the internal
registry. Inlining the tag here to buy coverage would make this
Dockerfile the outlier instead, against Rails-generated boilerplate.
Ruby bumps stay a manual, human-decided step.

.github/dependabot.yml

@@ -23,9 +23,9 @@ updates:
       exclude:
         - "brakeman"
 
-  - package-ecosystem: docker
-    directory: "/"
-    schedule:
-      interval: weekly
-    cooldown:
-      default-days: 7
+  # No docker ecosystem. Our base image tag is a build arg
+  # (`FROM ruby:$RUBY_VERSION-slim`), which the updater cannot read, so the
+  # entry only ever matched nothing — worse than no entry, because it looked
+  # like coverage. Ruby base image bumps are a manual, reviewed step here, as
+  # they are across the rest of the fleet. Keep Dockerfile, Dockerfile-export
+  # and .ruby-version on the same version by hand.