feat: dump build command when --out is specified (railwayapp#1115)

* feat: dump build command when --out is specified

the build command contains a lot of values which are generated from the nixpacks
build process. Without these dumped in the --out directory it's hard to debug
and replicate what's going on with a build.

* fmt

* fix static analysis error

src/nixpacks/builder/docker/docker_image_builder.rs

@@ -37,6 +37,18 @@ fn get_output_dir(app_src: &str, options: &DockerBuilderOptions) -> Result<Outpu
     }
 }
 
+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(),
+        args.join(" ")
+    )
+}
+
 use async_trait::async_trait;
 
 #[async_trait]
@@ -78,10 +90,17 @@ impl ImageBuilder for DockerImageBuilder {
         plan.write_supporting_files(&self.options, env, &output)
             .context("Writing supporting files")?;
 
+        let mut docker_build_cmd = self.get_docker_build_cmd(plan, name.as_str(), &output)?;
+
+        if self.options.out_dir.is_some() {
+            let command_path = output.get_absolute_path("build.sh");
+            File::create(command_path.clone()).context("Creating command.sh file")?;
+            fs::write(command_path, command_to_string(&docker_build_cmd))
+                .context("Write command")?;
+        }
+
         // Only build if the --out flag was not specified
         if self.options.out_dir.is_none() {
-            let mut docker_build_cmd = self.get_docker_build_cmd(plan, name.as_str(), &output)?;
-
             // Execute docker build
             let build_result = docker_build_cmd.spawn()?.wait().context("Building image")?;
             if !build_result.success() {