[WIP] Add Dockerfile to generate the Nulecule index into an image

[vpavlin]

This PR adds Dockerfile to the root of nulecule-library repository. This Dockerfile uses new feature of Atomic App I am also proposing - generation of the Index. When an Automated Build is set up, this Dockerfile will result in an image containing metadata for all Nulecule apps/components residing in this repository.

Kudos to @dustymabe for the idea of shipping the Index in an image:)

You can see the example image on Docker Hub as nulecule/index

$ docker run --rm nulecule/index
This image contains index information about Nulecule applications stored in nulecule-library

[vpavlin]

1. Image naming

I would also like to propose one guideline point as a part of this PR - names of the directories in this repository should match names of the images created from the Nulecules in them.

I think we already do this "by default", but having it written would be good..
f.e.

mariadb-fedora-atomicapp -> projectatomic/mariadb-fedora-atomicapp
wordpress-centos7-atomicapp -> projectatomic/wordpress-centos7-atomicapp

2. External Index Additions

Another thing is that there might be projects which would rather maintain their own repository for their apps/components, but it would be beneficial for us to have them in the Index.

This could be then solved by a similar yaml files @kbsingh proposed in cccp project earlier.

F.E. (Just a brain dump..)

$ more remote_entries.yaml
- id: postgresql-centos7-atomicapp
  git_url: https://github.com/vpavlin/guestbook-pgsql
  git_path: postgresql-centos7-atomicapp
  image_name: vpavlin/postgresql-centos7-atomicapp

Which can be easily incorporated in index during atomicapp index generate action.

[goern]

hmm, now big is that image carrying a ascii file?

[vpavlin]

Due to a Docker layring overhead...300 KB (cause I assume you already have the atomicapp base image pulled)

[goern]

nice answer :)

[goern]

should this be projectatomic/atomicapp ? .. after the merge projectatomic/atomicapp#379

[vpavlin]

Yes, definitely. I did it this way so that people can try it right away without having to build any images etc... Also, this is just a POC, I'm happy to pass the code to anybody who wants to carry on with it;)