New feature: Add --build-secret argument

[KarstenB]

In our CI pipeline, we use build secrets to inject secrets for our internal registries into our Dockerfile. However, the devcontainer CLI currently doesn't support using those secrets from within a feature. This limitation exists because the synthesized Dockerfile for building features requires additional parameters like --mount=type=secret,id=aws.

I created a PR that addresses this issue and allows providing secrets to features in the same way as with a regular docker buildx command. Here's an example of how it works:

In the feature install.sh:

#!/bin/bash
set -e
if [ -f /run/secrets/compose_file ]; then
    echo "Contents of secret file:"
    cat /run/secrets/compose_file
    echo "Writing secret file contents to /secret_file.txt"
    cat /run/secrets/compose_file > /secret_file.txt
else
    echo "Secret file not found!"
    exit 1
fi

On the terminal:

devcontainer build --workspace-folder test-build-secrets --build-secret id=compose_file,src=./my_cred.txt --image-name test-build-secrets-image --no-cache
docker run --rm --entrypoint="cat" test-build-secrets-image /secret_file.txt

This will build the devcontainer with the local feature and print out the secret content.

Note: Writing the secret to a file is obviously the opposite of what one would actually do with the build-secret flag, but it demonstrates that the secret can be read and used just like with regular Docker secrets.