The jekyll image specified in the docs is incompatible with

### Current Behavior

The command to run the doc server via docker in DEVELOPING.md uses jekyll:3, which is incompatible with the current dependency: public_suffix.

### Steps to Reproduce

1.
The docker command fails
```
$ docker run --rm -it --name jekyll -p "127.0.0.1:4000:4000" -v "$(pwd)/docs:/srv/jekyll:Z" jekyll/jekyll:3 jekyll serve

Warning: the running version of Bundler (2.0.2) is older than the version that created the lockfile (2.3.25). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
Fetching gem metadata from https://rubygems.org/............
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
public_suffix-6.0.1 requires ruby version >= 3.0, which is incompatible with the current version, ruby 2.6.3p62
```

### Expected Behavior

```
Fetching gem metadata from https://rubygems.org/...........
Using bundler 2.3.25
Fetching public_suffix 6.0.1
Using colorator 1.1.0
Fetching concurrent-ruby 1.3.4
Installing public_suffix 6.0.1
Installing concurrent-ruby 1.3.4
...
```

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this defect was already reported