Fix docker setup for wagtail guide

[VictoriaAjala]

This PR fixes issues I encountered when setting up wagtail with docker.

Without installing the necessary dependency with

RUN apt-get install -y gettext 

the make docker-init step fails with the error below:

[VictoriaAjala]

@lb- @thibaudcolas

[lb-]

I do not use docker, so I am not the best to review this - but I am sure someone in the team will see this and give feedback when available.

[saevarom]

I'd prefer not messing with the Heroku Dockerfile since that is used for deployments.
There is another PR #221 which defines a separate Dockerfile for local development and there we could add suport for gettext in the container.

So I would recommend merging #221 first and then think about supporting gettext in the local docker container. It would probably make the most sense to close this PR and create a new one after #221 is merged.

[VictoriaAjala]

Alright @saevarom, sounds good.

Closing this and creating a new PR when #221 has been merged.