docs: remove usage for jekyll-docker

The project [`jekyll-docker`](https://github.com/envygeeks/jekyll-docker/) has not been updated for more than a year and is not supported by `sass-cover-3.x` (`sass-embedded`) in Jekyll `4.3` due to the old OS version.

When developing sass code, we have to consider `sass-cover-2.x` (`sassc`) compatibility, which reduces web rendering performance and increases syntax maintenance costs.

Therefore, it makes no sense to maintain this awkward situation and the use of the deprecated image `jekyll/jekyll` should be removed from the documentation.

See also:

- envygeeks/jekyll-docker#363
- https://github.com/jekyll/jekyll-sass-converter#migrate-from-2x-to-3x

_posts/2019-08-09-getting-started.md

@@ -88,15 +88,6 @@ You may want to preview the site contents before publishing, so just run it by:
 $ bundle exec jekyll s
 ```
 
-Or run the site on Docker with the following command:
-
-```console
-$ docker run -it --rm \
-    --volume="$PWD:/srv/jekyll" \
-    -p 4000:4000 jekyll/jekyll \
-    jekyll serve
-```
-
 After a few seconds, the local service will be published at _<http://127.0.0.1:4000>_.
 
 ## Deployment
@@ -136,16 +127,6 @@ Go to the root of the source project, and build your site as follows:
 $ JEKYLL_ENV=production bundle exec jekyll b
 ```
 
-Or build the site on Docker:
-
-```console
-$ docker run -it --rm \
-    --env JEKYLL_ENV=production \
-    --volume="$PWD:/srv/jekyll" \
-    jekyll/jekyll \
-    jekyll build
-```
-
 Unless you specified the output path, the generated site files will be placed in folder `_site`{: .filepath} of the project's root directory. Now you should upload those files to the target server.
 
 [nodejs]: https://nodejs.org/