Add bash completion for docker-buildx plugin

[crazy-max]

- What I did

I was working on targets completion for the buildx bake command in docker/buildx#1674 and found out we don't have any bash completion for docker-buildx plugin.

- How I did it

- How to verify it

$ mkdir -p ~/.local/share/bash-completion/completions
$ cp ./contrib/completion/bash/docker ~/.local/share/bash-completion/completions/docker
$ _completion_loader docker
$ docker build [tab][tab]
build builder buildx
$ docker buildx [tab][tab]
bake create help inspect prune stop version build du imagetools ls rm use
$ docker buildx b [tab][tab]
bake build

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Merging #4089 (1d4f9fa) into master (cb5463a) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@ Coverage Diff @@## master #4089 +/- ##
==========================================
- Coverage 59.15% 59.14% -0.01% 
==========================================
Files 285 287 +2 Lines 24725 24741 +16 ==========================================
+ Hits 14627 14634 +7 - Misses 9212 9221 +9 
Partials 886 886 

[thaJeztah]

Guess it won't hurt to have it in this script, although we now also have the Cobra-generated completion in the CLI on master (and 23.0+) since #3429

[crazy-max]

Guess it won't hurt to have it in this script, although we now also have the Cobra-generated completion in the CLI on master (and 23.0+) since #3429

Yeah but it doesn't work for plugins afaik:

$ docker build [tab][tab]
build builder

Compose works because there is the bash completion available I guess.

[silvin-lubecki]

Maybe for a follow-up, but I think we should try to make it generic for plugins. We should call docker info only once and process the result, depending the plugin name.
It might be tricky though, so maybe for a follow-up 🤔

[crazy-max]

Thanks for your feedback. I will make it more generic to support all plugins. Keep you posted.