Secrets for containerEnv

Currently the --secrets-file sets the secret environment variables as `remoteEnv` which are only accessible by the vscode server process. A support for setting secrets as `containerEnv` shall ensure that all the running processes inside the devcontainer can access them.

At present, if I want to pass secrets at runtime I would have to pass them in `containerEnv` in `devcontainer.json` file, which is insecure as they are committed to the git repository. As a workaround, I make a docker compose file and pass the `env_file` there and use it for spinning up the devcontainer.