Previously, `CODER_WORKSPACE_AGENT_NAME` would always be passed as the
dev container name.
This is invalid for the following scenarios:
- The dev container is specified in terraform
- The dev container has a name customization
This change now runs `ReadConfig` twice. The first read is to extract a
name (if present), from the `devcontainer.json`. The second read will
then use the name we have stored for the dev container (so this could be
either the customization, terraform resource name, or container name).