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/tun.go

@@ -0,0 +1,21 @@
+package openvpn
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/qdm12/private-internet-access-docker/internal/constants"
+)
+
+// CheckTUN checks the tunnel device is present and accessible
+func (c *configurator) CheckTUN() error {
+	c.logger.Info("%s: checking for device %s", logPrefix, constants.TunnelDevice)
+	f, err := c.openFile(string(constants.TunnelDevice), os.O_RDWR, 0)
+	if err != nil {
+		return fmt.Errorf("TUN device is not available: %w", err)
+	}
+	if err := f.Close(); err != nil {
+		c.logger.Warn("Could not close TUN device file: %s", err)
+	}
+	return nil
+}