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>

Corefile

@@ -0,0 +1,3 @@
+.:53 {
+    forward . /etc/resolv.conf
+}

Dockerfile.multiarch

@@ -15,6 +15,9 @@ RUN --mount=type=cache,target=/root/.cache/go-build \
 FROM docker/buildx-bin:${BUILDX_VERSION} as buildx-bin
 FROM docker:${DOCKER_VERSION}
 
+RUN apk --update --no-cache add coredns
+
+COPY --from=build /src/Corefile /etc/coredns/Corefile
 COPY --from=buildx-bin /buildx /usr/libexec/docker/cli-plugins/docker-buildx
 COPY --from=build /src/plugin-docker-buildx /bin/plugin-docker-buildx
 

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
+}

plugin/impl.go

@@ -209,6 +209,17 @@ func (p *Plugin) writeBuildkitConfig() error {
 func (p *Plugin) Execute() error {
 	// start the Docker daemon server
 	if !p.settings.Daemon.Disabled {
+		// If no custom DNS value set start internal DNS server
+		if len(p.settings.Daemon.DNS.Value()) == 0 {
+			ip, err := getContainerIP()
+			if err != nil {
+				logrus.Warnf("error detecting IP address: %v", err)
+			} else if ip != "" {
+				p.startCoredns()
+				p.settings.Daemon.DNS.Set(ip)
+			}
+		}
+
 		p.startDaemon()
 	}