Remove unused Docker build environment

Author: andrew

.github/dependabot.yml

@@ -15,11 +15,3 @@ updates:
     open-pull-requests-limit: 5
     cooldown:
       default-days: 7
-
-  - package-ecosystem: docker
-    directory: /
-    schedule:
-      interval: weekly
-    open-pull-requests-limit: 5
-    cooldown:
-      default-days: 7

Dockerfile

@@ -86,8 +86,6 @@ The native formats (Markdown, Org-mode) need no external tools. For the others:
 | rst2html | `pip install docutils` or `apt install python3-docutils` | reStructuredText |
 | pod2html | Included with Perl | Pod |
 
-The included `Dockerfile` builds an image with all optional tools installed.
-
 ## Benchmarks
 
 ```