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/openvpn/auth.go

@@ -0,0 +1,23 @@
+package openvpn
+
+import (
+	"github.com/qdm12/golibs/files"
+	"github.com/qdm12/private-internet-access-docker/internal/constants"
+)
+
+// WriteAuthFile writes the OpenVPN auth file to disk with the right permissions
+func (c *configurator) WriteAuthFile(user, password string, uid, gid int) error {
+	authExists, err := c.fileManager.FileExists(string(constants.OpenVPNAuthConf))
+	if err != nil {
+		return err
+	} else if authExists { // in case of container stop/start
+		c.logger.Info("%s: %s already exists", logPrefix, constants.OpenVPNAuthConf)
+		return nil
+	}
+	c.logger.Info("%s: writing auth file %s", logPrefix, constants.OpenVPNAuthConf)
+	return c.fileManager.WriteLinesToFile(
+		string(constants.OpenVPNAuthConf),
+		[]string{user, password},
+		files.FileOwnership(uid, gid),
+		files.FilePermissions(0400))
+}