Change docs to use --only main syntax, and add missing argument to Dockerfile example

[shawnz]

This PR describes a suggestion which I believe improves upon two small issues I have with the docs regarding the --without dev/--only main option for the poetry install command.

1. The Commands page disagrees with the deprecation warning when using the deprecated --no-dev option: the Commands page suggests using --without dev instead, but the deprecation warning suggests using --only main here:

   poetry/src/poetry/console/commands/group_command.py

   Line 86 in ed1dcdd

   ("no-dev", "only", MAIN_GROUP),

   To resolve this, I updated the Commands page to suggest both options. Personally, I think --only main is preferrable overall because it works even if the dev dependency group doesn't exist, but it might be helpful for readers to know that both options are viable.

2. The FAQ entry about avoiding Docker cache busting doesn't apply either of the --without dev or --only main options to the first install statement, but rather only the second install statement. Based on my testing, this causes dev dependencies to get installed, which I think is not intended.

   To resolve this I added --only main to the first install statement, and also changed the second install statement to use the --only main syntax rather than the --without dev syntax for the reasons described above.

[shawnz]

Based on an initial positive response that --only main is preferable here, I just added a follow up commit which swaps the order of all the examples so that --only main is suggested first before --without dev

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