support agent forwarding

missed error check

add debugging

fix empty val for SSHAgent

fix flag type

remove []

debug

base64 encode ssh key

fix

remove debug output

code cleanup

Update docker.go

docker.go

@@ -1,8 +1,10 @@
 package docker
 
 import (
+	"encoding/base64"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -11,6 +13,11 @@ import (
 	"time"
 )
 
+const (
+	SSHAgentSockPath     = "/tmp/drone-ssh-agent-sock"
+	SSHPrivateKeyFromEnv = "SSH_KEY"
+)
+
 type (
 	// Daemon defines Docker daemon parameters.
 	Daemon struct {
@@ -107,6 +114,7 @@ type (
 
 // Exec executes the plugin step
 func (p Plugin) Exec() error {
+
 	// start the Docker daemon server
 	if !p.Daemon.Disabled {
 		p.startDaemon()
@@ -180,6 +188,13 @@ func (p Plugin) Exec() error {
 		cmds = append(cmds, commandPull(img))
 	}
 
+	// setup for using ssh agent (https://docs.docker.com/develop/develop-images/build_enhancements/#using-ssh-to-access-private-data-in-builds)
+
+	if len(p.Build.SSHAgent) > 0 {
+		fmt.Printf("ssh agent set to \"%s\"\n", p.Build.SSHAgent)
+		cmds = append(cmds, commandSSHAgentForwardingSetup(p.Build)...)
+	}
+
 	cmds = append(cmds, commandBuild(p.Build)) // docker build
 
 	for _, tag := range p.Build.Tags {
@@ -329,8 +344,8 @@ func commandBuild(build Build) *exec.Cmd {
 	if build.Platform != "" {
 		args = append(args, "--platform", build.Platform)
 	}
-	if build.SSHAgent != "" {
-		args = append(args, "--ssh", build.SSHAgent)
+	for _, sshagent := range build.SSHAgent {
+		args = append(args, "--ssh", sshagent)
 	}
 
 	if build.AutoLabel {
@@ -357,8 +372,8 @@ func commandBuild(build Build) *exec.Cmd {
 		}
 	}
 
-	// we need to enable buildkit, for secret support
-	if build.Secret != "" || len(build.SecretEnvs) > 0 || len(build.SecretFiles) > 0 {
+	// we need to enable buildkit, for secret support and ssh agent support
+	if build.Secret != "" || len(build.SecretEnvs) > 0 || len(build.SecretFiles) > 0 || len(build.SSHAgent) > 0 {
 		os.Setenv("DOCKER_BUILDKIT", "1")
 	}
 	return exec.Command(dockerExe, args...)
@@ -511,6 +526,40 @@ func commandRmi(tag string) *exec.Cmd {
 	return exec.Command(dockerExe, "rmi", tag)
 }
 
+func commandSSHAgentForwardingSetup(build Build) []*exec.Cmd {
+	cmds := make([]*exec.Cmd, 0)
+	if err := writeSSHPrivateKey(); err != nil {
+		log.Fatalf("unable to setup ssh agent forwarding: %s", err)
+	}
+	os.Setenv("SSH_AUTH_SOCK", SSHAgentSockPath)
+	cmds = append(cmds, exec.Command("ssh-agent", "-a", SSHAgentSockPath))
+	cmds = append(cmds, exec.Command("ssh-add"))
+	return cmds
+}
+
+func writeSSHPrivateKey() error {
+	privateKeyBase64 := os.Getenv(SSHPrivateKeyFromEnv)
+	if privateKeyBase64 == "" {
+		return fmt.Errorf("%s must be defined and contain the base64 encoded private key to use for ssh agent forwarding", SSHPrivateKeyFromEnv)
+	}
+	var err error
+	privateKey, err := base64.StdEncoding.DecodeString(privateKeyBase64)
+	if err != nil {
+		return fmt.Errorf("unable to base64 decode private key")
+	}
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return fmt.Errorf("unable to determine home directory: %s", err)
+	}
+	if err := os.MkdirAll(filepath.Join(home, ".ssh"), 0700); err != nil {
+		return fmt.Errorf("unable to create .ssh directory: %s", err)
+	}
+	if err := os.WriteFile(filepath.Join(home, ".ssh", "id_rsa"), privateKey, 0400); err != nil {
+		return fmt.Errorf("unable to write ssh key: %s", err)
+	}
+	return nil
+}
+
 // trace writes each command to stdout with the command wrapped in an xml
 // tag so that it can be extracted and displayed in the logs.
 func trace(cmd *exec.Cmd) {