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/dns/command.go

@@ -0,0 +1,40 @@
+package dns
+
+import (
+	"fmt"
+	"io"
+	"strings"
+
+	"github.com/qdm12/private-internet-access-docker/internal/constants"
+)
+
+func (c *configurator) Start(verbosityDetailsLevel uint8) (stdout io.ReadCloser, err error) {
+	c.logger.Info("%s: starting unbound", logPrefix)
+	args := []string{"-d", "-c", string(constants.UnboundConf)}
+	if verbosityDetailsLevel > 0 {
+		args = append(args, "-"+strings.Repeat("v", int(verbosityDetailsLevel)))
+	}
+	// Only logs to stderr
+	_, stdout, _, err = c.commander.Start("unbound", args...)
+	return stdout, err
+}
+
+func (c *configurator) Version() (version string, err error) {
+	output, err := c.commander.Run("unbound", "-V")
+	if err != nil {
+		return "", fmt.Errorf("unbound version: %w", err)
+	}
+	for _, line := range strings.Split(output, "\n") {
+		if strings.Contains(line, "Version ") {
+			words := strings.Fields(line)
+			if len(words) < 2 {
+				continue
+			}
+			version = words[1]
+		}
+	}
+	if version == "" {
+		return "", fmt.Errorf("unbound version was not found in %q", output)
+	}
+	return version, nil
+}