Debug custom targets, and a Docker image to build and develop in

- 🐞 on a custom target without an executable set debugs the program its
  command runs, as the build system has it (Ninja's edge, a Makefile's
  recipe): its arguments, directory and cmake -E env variables; what the
  target depends on is built first, not the target itself
- Debug for custom targets in the status bar, the Targets view and the
  code lenses above add_custom_target
- Dockerfile (Node, CMake, Ninja, g++, GDB), scripts/docker.sh to run a
  command in it, a dev container, and example/ to try the extension on

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

example/CMakeLists.txt

@@ -0,0 +1,25 @@
+# A small project to try the extension on: F5 "Run Extension (example)".
+cmake_minimum_required(VERSION 3.20)
+project(example CXX)
+
+set(CMAKE_CXX_STANDARD 17)
+
+add_library(greet src/greet.cpp)
+target_include_directories(greet PUBLIC src)
+
+add_executable(hello src/main.cpp)
+target_link_libraries(hello PRIVATE greet)
+
+enable_testing()
+add_executable(greet_test tests/greet_test.cpp)
+target_link_libraries(greet_test PRIVATE greet)
+add_test(NAME greet COMMAND greet_test)
+
+# Custom targets: ▶ builds them (runs their command), 🐞 debugs the program their command runs.
+add_custom_target(run_hello
+  COMMAND hello CMake
+  WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+  COMMENT "Running hello")
+add_custom_target(run_hello_env
+  COMMAND ${CMAKE_COMMAND} -E env GREETING=hi $<TARGET_FILE:hello> "with spaces"
+  COMMAND ${CMAKE_COMMAND} -E echo done)

example/src/greet.cpp

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+#include "greet.h"
+
+std::string greet(const std::string& name) {
+  return "Hello, " + name + "!";
+}

example/src/greet.h

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+#pragma once
+#include <string>
+
+std::string greet(const std::string& name);

example/src/main.cpp

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+#include <iostream>
+#include "greet.h"
+
+int main(int argc, char** argv) {
+  std::cout << greet(argc > 1 ? argv[1] : "world") << std::endl;
+  return 0;
+}

example/tests/greet_test.cpp

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+#include <iostream>
+#include "greet.h"
+
+int main() {
+  if (greet("CMake") != "Hello, CMake!") {
+    std::cerr << "unexpected greeting: " << greet("CMake") << std::endl;
+    return 1;
+  }
+  return 0;
+}