aider: Added check for existing Docker image before building.

# Context:
USER: in `build_docker` check if the image is already built and return
ASSISTANT: To check if the Docker image is already built and return if it exists, we can use the `docker images` command and parse its output. Here's how to modify the `build_docker` function:

```python
benchmark/benchmark.py
<<<<<<< ORIGINAL
def build_docker():
    command = [
        "docker",
        "build",
        "--quiet",
        "-t",
        "benchmark",
        "-f",
        "benchmark/Dockerfile",
        "/dev/null",
    ]
    print(" ".join(command))

    try:
        result = subprocess.run(
            command,
            stdout=subprocess.PIPE,
            stderr=subprocess.STDOUT,
            text=True,
        )
        res = result.stdout
        print(res)

    except subprocess.CalledProcessError as e:
        res = f"Failed to build Docker image: {e.output}"

    return res
=======
def build_docker():
    image_name = "benchmark"
    check_command = ["docker", "images", "-q", image_name]
    check_result = subprocess.run(check_command, stdout=subprocess.PIPE, text=True)
    if check_result.stdout.strip():
        print(f"Docker image '{image_name}' already exists, skipping build.")
        return

    command = [
        "docker",
        "build",
        "--quiet",
        "-t",
        image_name,
        "-f",
        "benchmark/Dockerfile",
        "/dev/null",
    ]
    print(" ".join(command))

    try:
        result = subprocess.run(
            command,
            stdout=subprocess.PIPE,
            stderr=subprocess.STDOUT,
            text=True,
        )
        res = result.stdout
        print(res)

    except subprocess.CalledProcessError as e:
        res = f"Failed to build Docker image: {e.output}"

    return res
>>>>>>> UPDATED
```

This modification adds a check for the existence of the Docker image before attempting to build it. If the image already exists, the function prints a message and returns early.

benchmark/benchmark.py

@@ -386,12 +386,19 @@ def run_unit_tests(testdir, history_fname):
 
 
 def build_docker():
+    image_name = "benchmark"
+    check_command = ["docker", "images", "-q", image_name]
+    check_result = subprocess.run(check_command, stdout=subprocess.PIPE, text=True)
+    if check_result.stdout.strip():
+        print(f"Docker image '{image_name}' already exists, skipping build.")
+        return
+
     command = [
         "docker",
         "build",
         "--quiet",
         "-t",
-        "benchmark",
+        image_name,
         "-f",
         "benchmark/Dockerfile",
         "/dev/null",