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/pia/download.go

@@ -0,0 +1,61 @@
+package pia
+
+import (
+	"archive/zip"
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"strings"
+
+	"github.com/qdm12/private-internet-access-docker/internal/constants"
+	"github.com/qdm12/private-internet-access-docker/internal/models"
+)
+
+func (c *configurator) DownloadOvpnConfig(encryption models.PIAEncryption,
+	protocol models.NetworkProtocol, region models.PIARegion) (lines []string, err error) {
+	c.logger.Info("%s: downloading openvpn configuration files", logPrefix)
+	URL := buildZipURL(encryption, protocol)
+	content, status, err := c.client.GetContent(URL)
+	if err != nil {
+		return nil, err
+	} else if status != 200 {
+		return nil, fmt.Errorf("HTTP Get %s resulted in HTTP status code %d", URL, status)
+	}
+	filename := fmt.Sprintf("%s.ovpn", region)
+	fileContent, err := getFileInZip(content, filename)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", URL, err)
+	}
+	lines = strings.Split(string(fileContent), "\n")
+	return lines, nil
+}
+
+func buildZipURL(encryption models.PIAEncryption, protocol models.NetworkProtocol) (URL string) {
+	URL = string(constants.PIAOpenVPNURL) + "/openvpn"
+	if encryption == constants.PIAEncryptionStrong {
+		URL += "-strong"
+	}
+	if protocol == constants.TCP {
+		URL += "-tcp"
+	}
+	return URL + ".zip"
+}
+
+func getFileInZip(zipContent []byte, filename string) (fileContent []byte, err error) {
+	contentLength := int64(len(zipContent))
+	r, err := zip.NewReader(bytes.NewReader(zipContent), contentLength)
+	if err != nil {
+		return nil, err
+	}
+	for _, f := range r.File {
+		if f.Name == filename {
+			readCloser, err := f.Open()
+			if err != nil {
+				return nil, err
+			}
+			defer readCloser.Close()
+			return ioutil.ReadAll(readCloser)
+		}
+	}
+	return nil, fmt.Errorf("%s not found in zip archive file", filename)
+}