gateway spec: support envfile

[blueyed]

I think it would be good to have support for "envfile:PATH" next to "env:NAME=value" with the gateway spec (https://codespeak.net/execnet/basics.html#execnet.makegateway).

This format should be the same as with Docker's --env-file, i.e. a NAME=value pair per line.

[blueyed]

This is actually required to have env values containing "//" apparently:

neither key nor value are allowed to contain "//"

execnet/execnet/xspec.py

Lines 6 to 13 in d3384d3

	class XSpec:
	""" Execution Specification: key1=value1//key2=value2 ...
	* keys need to be unique within the specification scope
	* neither key nor value are allowed to contain "//"
	* keys are not allowed to contain "="
	* keys are not allowed to start with underscore
	* if no "=value" is given, assume a boolean True value
	"""

[blueyed]

I had tried to work around this using "//env:${(j-//env:-)${(@f)"$(<build/envfile)"}}" (zsh)

I ended up putting it into ~/.ssh/environment (requires -o PermitUserEnvironment=yes with sshd)

[RonnyPfannschmidt]

closing as super-seeded by #175
instead of implementing the https://docs.docker.com/compose/env-file/ format, its safer to use a existing implementation