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/settings/firewall.go

@@ -0,0 +1,34 @@
+package settings
+
+import (
+	"net"
+	"strings"
+
+	"github.com/qdm12/private-internet-access-docker/internal/params"
+)
+
+// Firewall contains settings to customize the firewall operation
+type Firewall struct {
+	AllowedSubnets []net.IPNet
+}
+
+func (f *Firewall) String() string {
+	var allowedSubnets []string
+	for _, net := range f.AllowedSubnets {
+		allowedSubnets = append(allowedSubnets, net.String())
+	}
+	settingsList := []string{
+		"Firewall settings:",
+		"Allowed subnets: " + strings.Join(allowedSubnets, ", "),
+	}
+	return strings.Join(settingsList, "\n |--")
+}
+
+// GetFirewallSettings obtains firewall settings from environment variables using the params package.
+func GetFirewallSettings(params params.ParamsReader) (settings Firewall, err error) {
+	settings.AllowedSubnets, err = params.GetExtraSubnets()
+	if err != nil {
+		return settings, err
+	}
+	return settings, nil
+}