chore: Capitalize Dockerfile to add support of tools (#2045)

For example, there is not syntax highlighting in GitHub, because file is
named `dockerfile` (and not Dockerfile), for example:
https://github.com/XRPLF/clio/blob/release/2.4.0/docker/ci/dockerfile

I also grep'ed dockerfile in repo and fixed some other places

.github/workflows/update_docker_ci.yml

@@ -8,7 +8,7 @@ on:
   push:
     branches: [develop]
     paths:
-      # CI image must update when either its dockerfile changes
+      # CI image must update when either its Dockerfile changes
       # or any compilers changed and were pushed by hand
       - "docker/ci/**"
       - "docker/compilers/**"