Update documentation for elyra development image

[akchinSTC]

Fixes #2859

What changes were proposed in this pull request?

How was this pull request tested?

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:

[kevin-bates]

LGTM - thanks @akchinSTC.

[ptitzler]

I've pushed a change to the README that fixes markdown rendering issues, such as indentation and numbering. I've also added <...> around placeholders everywhere to improve clarity, e.g. image_name => <image_name>. It would have taken me just as long to describe the issues ...

Before:

After:

[ptitzler]

In my environment the container won't start:

$ docker images
REPOSITORY                         TAG       IMAGE ID       CREATED         SIZE
image_name                         tag       5c36ea1ea69c   9 minutes ago   1.23GB

$ docker run -t image_name:tag -v /Users/patti/temp/elyra/:/dev/elyra -p 8888:8888
[FATAL tini (7)] exec -v failed: No such file or directory

$ ls /Users/patti/temp/elyra/
CONTRIBUTING.md		conftest.py		package.json		test_requirements.txt
...

[kiersten-stokes]

LGTM!