Followup to #1564 (cli plugins)

[ijc]

From #1564 (comment):

• Basic framework for writing and running CLI plugins #1564 (comment) handling of docker someplugin --version. The --version / -v flag is handled by the plugin root-command, and therefore shows the docker version information (Docker version 19.03.0-dev, build c8f1f1ee). We should fix this to be handled by the plugin.
• Related to the above; consider making --version / -v part of the pluging "API", so that a plugin can advertise its version (both when calling the plugin directly (docker-myplugin --version), and when calling through the docker cli (docker myplugin --version). Perhaps we should add a standard function and/or define a standard format (docker-myplugin version X, commit deadbeef (ACME, Inc.))
• Basic framework for writing and running CLI plugins #1564 (comment) install location for plugins; choose between ~/.docker/plugins or ~/.docker/cli-plugins → See issue Should cli plugins install to ~/.docker/plugins #1677 for discussion.
• Basic framework for writing and running CLI plugins #1564 (comment) decide on the output format in docker --help: make plugin-command stand out from built-in commands, or make them appear as regular commands with a non-obtrusive indicator (*) → See PR Reformat plugin's vendor position and add version on --help #1675.
• Basic framework for writing and running CLI plugins #1564 (comment) decide on the location to show version and vendor for plugins in in docker info → See PR Reformat the output of CLI plugins in docker system info #1689.
• Basic framework for writing and running CLI plugins #1564 (comment) small nit: use displayutils.Ellipsis() instead of a custom implementation. Obsoleted by PR Reformat plugin's vendor position and add version on --help #1675.
• Basic framework for writing and running CLI plugins #1564 (comment) small nit: hardening config.Path() to disallow directory traversal
• Basic framework for writing and running CLI plugins #1564 (comment) small nit: fix usage output of the sample plugin

Others:

• plugins shuld be able to indicate if they are "management" vs not commands in their metadata and be listed accordingly in e.g. docker help → See PR Show plugins as Management commands #1684.
• plugins should have a field in config.json for their use, see Configuration in design → See PR Add a field to the config file for plugin use. #1652.
• plugins should call back to main cli using dial-stdio to connect to engine. From CLI Plugins Design #1534 (comment) → See PR cli-plugins: use system dial-stdio to contact the engine. #1654 .

[thaJeztah]

noticed another thing; open a separate issue for that #1682