Rewrite of the entrypoint in Golang (#71)

- General improvements
    - Parallel download of only needed files at start
    - Prettier console output with all streams merged (openvpn, unbound, shadowsocks etc.)
    - Simplified Docker final image
    - Faster bootup
- DNS over TLS
    - Finer grain blocking at DNS level: malicious, ads and surveillance
    - Choose your DNS over TLS providers
    - Ability to use multiple DNS over TLS providers for DNS split horizon
    - Environment variables for DNS logging
    - DNS block lists needed are downloaded and built automatically at start, in parallel
- PIA
    - A random region is selected if the REGION parameter is left empty (thanks @rorph for your PR)
    - Routing and iptables adjusted so it can work as a Kubernetes pod sidecar (thanks @rorph for your PR)

internal/shadowsocks/conf.go

@@ -0,0 +1,49 @@
+package shadowsocks
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/qdm12/golibs/files"
+	"github.com/qdm12/private-internet-access-docker/internal/constants"
+)
+
+func (c *configurator) MakeConf(port uint16, password string, uid, gid int) (err error) {
+	c.logger.Info("%s: generating configuration file", logPrefix)
+	data := generateConf(port, password)
+	return c.fileManager.WriteToFile(
+		string(constants.ShadowsocksConf),
+		data,
+		files.FileOwnership(uid, gid),
+		files.FilePermissions(0400))
+}
+
+func generateConf(port uint16, password string) (data []byte) {
+	conf := struct {
+		Server       string            `json:"server"`
+		User         string            `json:"user"`
+		Method       string            `json:"method"`
+		Timeout      uint              `json:"timeout"`
+		FastOpen     bool              `json:"fast_open"`
+		Mode         string            `json:"mode"`
+		PortPassword map[string]string `json:"port_password"`
+		Workers      uint              `json:"workers"`
+		Interface    string            `json:"interface"`
+		Nameserver   string            `json:"nameserver"`
+	}{
+		Server:   "0.0.0.0",
+		User:     "nonrootuser",
+		Method:   "chacha20-ietf-poly1305",
+		Timeout:  30,
+		FastOpen: false,
+		Mode:     "tcp_and_udp",
+		PortPassword: map[string]string{
+			fmt.Sprintf("%d", port): password,
+		},
+		Workers:    2,
+		Interface:  "tun",
+		Nameserver: "127.0.0.1",
+	}
+	data, _ = json.Marshal(conf)
+	return data
+}