Docs: Update README with alternative setup with Docker

Update README with alternative setup with Docker, detailing Docker-specific steps.

README.md

@@ -10,3 +10,41 @@
   - --custom-path pointing to the repo directory
   - --config pointing to the custom app.ini
 - Update your user preferences to set the new theme, or it will keep showing the old one.
+
+## Alternative setup with Docker
+
+- Initialize `docker-compose.yml` following Gitea's [basic instructions](https://docs.gitea.com/installation/install-with-docker#basics).
+- Install Gitea following Gitea's [installation instructions](https://docs.gitea.com/installation/install-with-docker#installation).
+- Edit `app.ini` in `[Gitea's root]/conf` to append the following:
+```
+[ui]
+DEFAULT_THEME = bthree-dark
+THEMES = bthree-dark
+```
+(Docker Gitea's root directory defaults to `/gitea/gitea/` from installation root.)
+- Clone this repository to Gitea's root directory, so that its contents are merged. (Gitea's _custom_ directory defaults to Gitea's root.)
+
+Gitea's root flat directory tree should look similar to the following, if done correctly:
+```
+.
+├── actions_artifacts
+├── actions_log
+├── attachments
+├── avatars
+├── conf
+├── gitea.db
+├── home
+├── indexers
+├── jwt
+├── log
+├── packages
+├── public
+├── queues
+├── README.md
+├── repo-archive
+├── repo-avatars
+├── sessions
+├── sphinx
+├── templates
+└── tmp
+```