fix: correct argument handling when setting the --app flag

When setting the `--app` flag, there was a special case for the config plugin. This plugin used to take the `--no-restart` flag as the first argument - and still did until this change - resulting in an invalid call to the plugin subcommands. What is worse is that this functionality carried over to all other plugins as they were rewritten in golang, and thus the issue spread without us knowing.

The fix is to support GNU flag parsing rules, which Golang does not do by default (Because Of Reasons™). Switching to `github.com/spf13/pflag` fixes this issue, and also allows positional arguments to be placed wherever, even in the middle of flags.

Closes #4255

plugins/proxy/go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/dokku/dokku/plugins/common v0.0.0-00010101000000-000000000000
 	github.com/dokku/dokku/plugins/config v0.0.0-00010101000000-000000000000
 	github.com/ryanuber/columnize v1.1.2-0.20190319233515-9e6335e58db3
+  github.com/spf13/pflag v1.0.5 // indirect
 )
 
 replace github.com/dokku/dokku/plugins/common => ../common

plugins/proxy/go.sum

@@ -6,3 +6,5 @@ github.com/joho/godotenv v1.2.0 h1:vGTvz69FzUFp+X4/bAkb0j5BoLC+9bpqTWY8mjhA9pc=
 github.com/joho/godotenv v1.2.0/go.mod h1:7hK45KPybAkOC6peb+G5yklZfMxEjkZhHbwpqxOKXbg=
 github.com/ryanuber/columnize v1.1.2-0.20190319233515-9e6335e58db3 h1:utdYOikI1XjNtTFGCwSM6OmFJblU4ld4gACoJsbadJg=
 github.com/ryanuber/columnize v1.1.2-0.20190319233515-9e6335e58db3/go.mod h1:sm1tb6uqfes/u+d4ooFouqFdy9/2g9QGwK3SQygK0Ts=
+github.com/spf13/pflag v1.0.5 h1:iy+VFUOCP1a+8yFto/drg2CJ5u0yRoB7fZw3DKv/JXA=
+github.com/spf13/pflag v1.0.5/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=

plugins/proxy/src/subcommands/subcommands.go

@@ -1,13 +1,14 @@
 package main
 
 import (
-	"flag"
 	"fmt"
 	"os"
 	"strings"
 
 	"github.com/dokku/dokku/plugins/common"
 	"github.com/dokku/dokku/plugins/proxy"
+
+	flag "github.com/spf13/pflag"
 )
 
 // main entrypoint to all subcommands