Docker command customization documentation

[bwateratmsft]

Adds documentation for the Docker command customization features. The content is adapted from microsoft/vscode-docker#1596 (comment).

@karolz-ms, @philliphoff, @BigMorty for FYI.

This takes care of microsoft/vscode-docker#1653.

[bwateratmsft]

@ghogen @gregvanl can you review? Thank you!

[scscgit]

I know this doesn't relate to the docs affected by this pull request, as they omit examples, but I want to mention for people who may come here that the microsoft/vscode-docker#1596 issue (which is closed and doesn't allow new comments), which is currently a leading google search result for VS Code tail (customization) issues, has a typo of "-tail" instead of "--tail", and that it makes more sense to also combine this with "--follow" to display new output in real time, so I think a better recommended customized value of "docker.commands.logs" (for people annoyed by being unable to view logs of long-running services) would be:

"docker logs --follow --tail 1000 ${containerId}"

(Personally I think it would be ideal if docs or even the Settings window had some popular & opinionated use-cases covered as a recommendation on top of just providing the default.)

[bwateratmsft]

@scscgit I fixed the typo on that GitHub issue, thanks! I think we should consider changing the default of that command to include --tail, so I opened microsoft/vscode-docker#2369.