Use env lib, add PROXY_PORT ENV variable

vendor/github.com/caarlos0/env/parsers/README.md

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+parsers
+=======
+This directory contains pre-built, custom parsers that can be used with `env.ParseWithFuncs`
+to facilitate the parsing of envs that are not basic types.
+
+Example Usage:
+
+```golang
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/url"
+
+	"github.com/caarlos0/env"
+	"github.com/caarlos0/env/parsers"
+)
+
+type config struct {
+	ExampleURL url.URL `env:"EXAMPLE_URL" envDefault:"https://google.com"`
+}
+
+func main() {
+	cfg := config{}
+
+	if err := env.ParseWithFuncs(&cfg, env.CustomParsers{
+		parsers.URLType: parsers.URLFunc,
+	}); err != nil {
+		log.Fatal("Unable to parse envs: ", err)
+	}
+
+	fmt.Printf("Scheme: %v Host: %v\n", cfg.ExampleURL.Scheme, cfg.ExampleURL.Host)
+}
+```

vendor/github.com/caarlos0/env/parsers/parsers.go

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+// Package parsers contains custom parser funcs for common, non-built-in types
+package parsers
+
+import (
+	"fmt"
+	"net/url"
+	"reflect"
+)
+
+var (
+	// URLType is a helper var that represents the `reflect.Type`` of `url.URL`
+	URLType = reflect.TypeOf(url.URL{})
+)
+
+// URLFunc is a basic parser for the url.URL type that should be used with `env.ParseWithFuncs()`
+func URLFunc(v string) (interface{}, error) {
+	u, err := url.Parse(v)
+	if err != nil {
+		return nil, fmt.Errorf("Unable to complete URL parse: %v", err)
+	}
+
+	return *u, nil
+}