Move docker base images (#2505)

Dependabot has been failing the last few weeks to update the base images
with dependabot (thought to be because they're in the `.github`
directory). Move them out into a new top-level `baseimages` directory to
see if dependabot will work again.

Author: pkwarren

.github/dependabot.yml

@@ -21,7 +21,7 @@ updates:
           - "*"
   # plugin base images
   - package-ecosystem: "docker"
-    directory: "/.github/docker"
+    directory: "/baseimages"
     schedule:
       interval: "daily"
     registries:

CONTRIBUTING.md

@@ -236,9 +236,9 @@ If the `fetcher` command opens a PR for a new version of an existing plugin, mos
 
 ### Updating Docker Base Images
 
-Docker base images are tracked in [.github/docker](.github/docker) and kept updated with Dependabot.
+Docker base images are tracked in [baseimages](baseimages) and kept updated with Dependabot.
 When new versions of a plugin are detected, they'll automatically be built with the latest base images.
-When creating a new plugin, ensure that it starts with the latest version of the base image in the `.github/docker` directory.
+When creating a new plugin, ensure that it starts with the latest version of the base image in the `baseimages` directory.
 
 ## Local Testing