fix: Support direct custom build script

e2e/assets/custom_canister/build.sh

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+echo CUSTOM_CANISTER2_BUILD_DONE

e2e/assets/custom_canister/dfx.json

@@ -6,6 +6,12 @@
       "candid": "main.did",
       "wasm": "main.wasm",
       "build": "echo CUSTOM_CANISTER_BUILD_DONE"
+    },
+    "custom2": {
+      "type": "custom",
+      "candid": "main.did",
+      "wasm": "main.wasm",
+      "build": "build.sh"
     }
   },
   "defaults": {

e2e/tests-dfx/build.bash

@@ -128,11 +128,14 @@ teardown() {
   install_asset custom_canister
   dfx_start
   dfx canister create --all
-  assert_command dfx build
+  assert_command dfx build custom
   assert_match "CUSTOM_CANISTER_BUILD_DONE"
+  assert_command dfx build custom2
+  assert_match "CUSTOM_CANISTER2_BUILD_DONE"
 
   dfx canister install --all
   assert_command dfx canister call custom fromQuery
+  assert_command dfx canister call custom2 fromQuery
 }
 
 @test "build succeeds with network parameter" {

src/dfx/src/lib/builders/custom.rs

@@ -13,8 +13,8 @@ use ic_types::principal::Principal as CanisterId;
 use serde::Deserialize;
 use slog::info;
 use slog::Logger;
-use std::path::PathBuf;
-use std::process::Stdio;
+use std::path::{Path, PathBuf};
+use std::process::{Command, Stdio};
 
 /// Set of extras that can be specified in the dfx.json.
 struct CustomBuilderExtra {
@@ -189,8 +189,16 @@ impl CanisterBuilder for CustomBuilder {
 
 fn run_command(args: Vec<String>, vars: &[super::Env<'_>]) -> DfxResult<()> {
     let (command_name, arguments) = args.split_first().unwrap();
-
-    let mut cmd = std::process::Command::new(command_name);
+    let command_path = Path::new(command_name);
+    let mut cmd = if arguments.is_empty()
+        && command_path.components().count() == 1
+        && command_path.exists()
+    {
+        // handle "command.sh" with no ./
+        Command::new(format!("./{command_name}"))
+    } else {
+        Command::new(command_name)
+    };
 
     cmd.args(arguments)
         .stdout(Stdio::inherit())