Fluentd-elasticsearch image with mongo plugin

Hello people of the future (including future self and search-engines).

Using the [Stable Kubernetes Helm chart](https://github.com/helm/charts/tree/master/stable/fluentd) of Fluentd-Elasticsearch, it is not possible to install `fluent-plugin-mongo` using the `values.yaml` definition [`plugins.pluginList`](https://github.com/helm/charts/blob/master/stable/fluentd/values.yaml#L44).
This happens as the plugin contains native components (that cannot be built as the existing container does not have building tools).

To fix that I created the following docker image, based on `gcr.io/google-containers/fluentd-elasticsearch`, which has the `fluent-plugin-mongo` installed, and ready to be used with `` and ``.

Here it is for everyone to use:
https://hub.docker.com/r/operatorequals/fluentd-elasticsearch