feat: add debug option to Docker build commands

This would enable an interative shell to be opened up if a build fails. This is still experimental
and I'm running into some issues using this with orbstack, so this PR is still very much a WIP.

orbstack/orbstack#1700

Without this option, the only way to debug a failing build is to `--out` the build, insert multi-stage
build commands in the dockerfile and then use `--target` to get in-progress container ID to play around
with, which is super lame. We need a better way.

src/main.rs

@@ -163,6 +163,10 @@ enum Commands {
         #[arg(long)]
         docker_cert_path: Option<String>,
 
+        /// Open an interactive shell if the build fails
+        #[arg(long)]
+        debug: bool,
+
         /// Enable writing cache metadata into the output image
         #[arg(long)]
         inline_cache: bool,

src/nixpacks/builder/docker/docker_image_builder.rs

@@ -36,11 +36,14 @@ fn get_output_dir(app_src: &str, options: &DockerBuilderOptions) -> Result<Outpu
     }
 }
 
+// TODO need to quote the output here otherwise the build command will fail with special char ENV vars
+
 fn command_to_string(command: &Command) -> String {
     let args = command
         .get_args()
         .map(|arg| arg.to_string_lossy())
         .collect::<Vec<_>>();
+
     format!(
         "{} {}",
         command.get_program().to_string_lossy(),
@@ -145,8 +148,16 @@ impl DockerImageBuilder {
 
         // Enable BuildKit for all builds
         docker_build_cmd.env("DOCKER_BUILDKIT", "1");
+
+        if self.options.debug {
+            docker_build_cmd.env("BUILDX_EXPERIMENTAL", "1");
+        }
 
         docker_build_cmd
+            .arg("buildx")
+            .arg("debug")
+            .arg("--invoke")
+            .arg("bash")
             .arg("build")
             .arg(&output.root)
             .arg("-f")

src/nixpacks/builder/docker/mod.rs

@@ -21,6 +21,7 @@ pub struct DockerBuilderOptions {
     pub cpu_quota: Option<String>,
     pub memory: Option<String>,
     pub verbose: bool,
+    pub debug: bool,
     pub docker_host: Option<String>,
     pub docker_tls_verify: Option<String>,
     pub docker_output: Option<String>,