Passing a Variable to copybara.sky in Dockerized Copybara

Hello,

I'm currently working on synchronizing two repositories (from repoA to repoB) using Copybara running in a Docker container. Below is the command I'm using to initiate the sync process:
```
docker_cmd = [
"docker run", "-v ~/.ssh:/root/.ssh", "-v ~/mongodb-bot.gitconfig:/root/.gitconfig",
f'-v "{current_dir}/copybara.sky":/usr/src/app/copy.bara.sky',
"-e COPYBARA_CONFIG='copy.bara.sky'", "-e COPYBARA_SUBCOMMAND='migrate'",
"-e COPYBARA_OPTIONS='-v'", "copybara copybara"
]
```
I've noticed that the name and email from ~/mongodb-bot.gitconfig are being utilized correctly during the sync to repoB. However, now I need to pass a GitHub Apps token to the destinationUrl in my copybara.sky configuration file.

I have tried hardcoding the token directly in the URL like this:

python
`destinationUrl = "https://x-access-token:@github.com/xxx/xx.git"`
And it works as expected. However, for security and flexibility reasons, I'd like to pass the token from my Python script to the copybara.sky file instead of hardcoding it.

I have tried to add it to my .gitconfig file like below, but it does not work.
```
[http]
extraheader = x-access-token:{token}
```

Is there a way to pass a variable (in this case, the GitHub Apps token) from my Python environment to the copybara.sky file? If so, could you please guide me on how to achieve this?