Add internal DNS server if no custom DNS is set (#45)

Fixes #44

Co-authored-by: Lauris BH <lauris@nix.lv>
Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/45
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lafriks@noreply.codeberg.org>
Co-committed-by: Lauris BH <lafriks@noreply.codeberg.org>

plugin/coredns.go

@@ -0,0 +1,48 @@
+package plugin
+
+import (
+	"io"
+	"net"
+	"os"
+	"os/exec"
+)
+
+func (p Plugin) startCoredns() {
+	cmd := exec.Command("coredns", "-conf", "/etc/coredns/Corefile")
+	if p.settings.Daemon.Debug {
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+	} else {
+		cmd.Stdout = io.Discard
+		cmd.Stderr = io.Discard
+	}
+	go func() {
+		trace(cmd)
+		cmd.Run()
+	}()
+}
+
+func getContainerIP() (string, error) {
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		return "", err
+	}
+	for _, i := range ifaces {
+		addrs, err := i.Addrs()
+		if err != nil {
+			return "", err
+		}
+		for _, addr := range addrs {
+			switch v := addr.(type) {
+			case *net.IPNet:
+				if v.IP.IsLoopback() {
+					continue
+				}
+				if v.IP.To4() != nil {
+					return v.IP.String(), nil
+				}
+			}
+		}
+	}
+	return "", nil
+}