Add docker to maintained CONTRIBUTING.md examples

Add the docker example, recently added, to the list of maintained theia
versions in CONTRIBUTING.md. The related PR simply forgot to amend this
specific part, that is also recent.

Signed-off-by: Marco Miller <marco.miller@ericsson.com>

CONTRIBUTING.md

@@ -112,6 +112,7 @@ more may be necessary depending on the case. Here is an example used to uplift f
    * Some bumps may then be required also in the extension; consider applying them.
 1. Uplift the version of each `@theia/` dependency in these `package.json` files, from `1.34.1` to `1.34.2` (replacing the former with the latter):
    * `./examples/browser/package.json`
+   * `./examples/docker/example-package.json`
    * `./examples/electron/package.json`
    * `./package.json`
    * `./theia-extensions/viewer-prototype/package.json`