inspect: add shell completion, improve flag-description for --type and improve validation

[thaJeztah]

• relates to Unclear what a type is when using docker inspect #6047

inspect: disable default (file) completion

Before this patch, flags and arguments would complete using filenames
from the current directory;

docker inspect --type <TAB>
AUTHORS       CONTRIBUTING.md             docs/             Makefile            SECURITY.md
...

docker inspect <TAB>

With this patch, no completion is provided;

docker inspect --type <TAB>
# no results

docker inspect <TAB>
# no results

inspect: add shell-completion for "--type" flag

With this patch:

docker inspect --type <TAB>
config     image    node    secret   task
container  network  plugin  service  volume

inspect: update flag description of "--type" flag

Before this patch:

docker inspect --help | grep '\-\-type'
      --type string     Return JSON for specified type

With this patch:

docker inspect --help | grep '\-\-type'
      --type string     Only inspect objects of the given type

inspect: improve (flag) validation

Produce an error if the --type flag was set, but an empty value
was passed.

Before this patch:

docker inspect --type "" foo
# json output

docker inspect --type unknown foo
"unknown" is not a valid value for --type

With this patch:

docker inspect --type "" foo
type is empty: must be one of "config", "container", "image", "network", "node", "plugin", "secret", "service", "task", "volume"

docker inspect --type unknown foo
unknown type: "unknown": must be one of "config", "container", "image", "network", "node", "plugin", "secret", "service", "task", "volume"

- Human readable description for the release notes

`docker inspect`: add shell completion, improve flag-description for `--type` and improve validation

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 48.14815% with 14 lines in your changes missing coverage. Please review.

Project coverage is 59.07%. Comparing base (25a1681) to head (52752f3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6052      +/-   ##
==========================================
+ Coverage   58.99%   59.07%   +0.08%     
==========================================
  Files         358      358              
  Lines       30004    30013       +9     
==========================================
+ Hits        17700    17731      +31     
+ Misses      11323    11301      -22     
  Partials      981      981              

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