pass variant macos support

[glensc]

looks like it's assumed that pass is to be used on Linux only.

however pass written in bash, a frontend to gpg, and therefore is portable and works fine on macos, there's even brew formula available.

however build system assumes it's for Linux:

==> make pass
Last 15 lines from /Users/glen/Library/Logs/Homebrew/docker-credential-helper/03.make:
2019-05-04 18:16:10 +0300

make
pass

mkdir -p bin
go build -o bin/docker-credential-pass pass/cmd/main_linux.go
pass/cmd/main_linux.go:5:2: build constraints exclude all Go files in /private/tmp/docker-credential-helper-20190504-69257-nmhjjp/docker-credential-helpers-0.6.2/src/github.com/docker/docker-credential-helpers/pass
make: *** [pass] Error 1

in source code there's only Linux source:

• https://github.com/docker/docker-credential-helpers/tree/v0.6.2/pass

should be trivial to port to other platforms (macOS i'm interested now)

[glensc]

note, this builds just fine:

==> cp pass/cmd/main_linux.go pass/cmd/main_darwin.go
==> cp pass/pass_linux.go pass/pass_darwin.go
==> go build -o bin/docker-credential-pass pass/cmd/main_darwin.go

[hutson]

Looks like @ellsclytn filed a pull request #169 to allow pass to build on MacOS.

I'm looking forward to this change making it easier for Homebrew/Linuxbrew users to use the Docker Credentials Helper pass package across platforms for greater credential security and consistency. (Homebrew/homebrew-core#39517)