Update documentation on containers and public container registries

[pacospace]

Signed-off-by: Francesco Murdaca fmurdaca@redhat.com

Fixes: #1035 updating Elyra documentation generalizing on container and container registries.

Developer's Certificate of Origin 1.1

   By making a contribution to this project, I certify that:

   (a) The contribution was created in whole or in part by me and I
       have the right to submit it under the Apache License 2.0; or

   (b) The contribution is based upon previous work that, to the best
       of my knowledge, is covered under an appropriate open source
       license and I have the right under that license to submit that
       work with modifications, whether created in whole or in part
       by me, under the same open source license (unless I am
       permitted to submit under a different license), as indicated
       in the file; or

   (c) The contribution was provided directly to me by some other
       person who certified (a), (b) or (c) and I have not modified
       it.

   (d) I understand and agree that this project and the contribution
       are public and that a record of the contribution (including all
       personal information I submit with it, including my sign-off) is
       maintained indefinitely and may be redistributed consistent with
       this project or the open source license(s) involved.

[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[akchinSTC]

Reads good to me..=)

[ptitzler]

See comments related to https://docs.docker.com/engine/reference/commandline/pull/#pull-from-a-different-registry

[ptitzler]

Could you add another example that illustrates what this would look like for a third party container registry e.g. quay.io/jdoe/my-image:1.0.0

[pacospace]

Updated in 453592b

[ptitzler]

LGTM