Support the --system flag in `pipenv run`

Is your feature request related to a problem? Please describe.

When using pipenv in a docker container you can install dependencies using the --system flag. However if you use pipenv run in the docker container it will automatically create a virtualenv and there is no way to stop this - this entails installing pip and wheel inside the container, which in specific environments may not be possible or may take a long time.

Describe the solution you’d like

Provide a --system flag to pipenv run that skips the creation of a virtualenv and instead uses the system default.

[orf]

All in all it’s just a bit inconsistent. You can take advantage of the Pipfile (or specifically the Pipfile.lock) to install system dependencies, but you cannot make use of [scripts] to run commands in the system interpreter for no clear reason.

Not to mention you might have a system command in [scripts] that doesn’t even depend on Python, perhaps one to build documentation with a go project. It would be nice to skip the install phase for this.

Having said that perhaps --system is a bad name for the flag, maybe --skip-virtualenv?

[orf]

Having the commands defined in the Pipfile has it’s advantages. We have a prod and dev command defined so our base docker image just executes pipenv run prod. This is quite nice as the commands are in a single place and anyone can get working, plus the docker image just needs to run a single command for all projects so less duplication.

Also in our case pipenv run dev executes pipenv run prod with some extra development specific arguments.