Debug FFmpeg in VSCode

1. Build & start docker container for building FFmpeg

## build docker image with Dockerfile, and start a container of this image
## you should use this.
docker compose up --build -d

##----------other commands------------
## build image only
docker compose --build

## start container only
docker compose up -d

## stop and remove container
docker compose down

## check with ps
docker compose ps

2. Debug FFmpeg

0. Download FFmpeg source code to src/ffmpeg Clone n6.1.1(tag) version for example, to check all versions on Official FFmpeg repo

   note: target folder MUST be src/ffmpeg

   git clone -b n6.1.1 https://git.ffmpeg.org/ffmpeg.git ./src/ffmpeg

1. Connect docker container in VSCode (2 options, either 1 or 2 is OK)

   1. Dev Container: Reopen in dev container within VSCode
   2. Remote Explorer: Attach docker container created in last step in VSCode and then Open /app folder as project root directory
      1. Install VSCode extensions in dev container
         1. Install C/C++ VSCode extension (for debugging)
         2. Install clangd VSCode extension (for code navigation)

2. Build FFmpeg

   /app/src/build.sh

3. Debug -> "Debug ffmpeg_g"