Improve dev container configuration to remove duplicate fields such as workspaceFolder and appPort

**Motivation**

To have consistent field values, easy for maintain and update.

For now, fields `workspaceFolder`, `workspaceMount` and `appPort` are duplicated in `.devcontainer.json`, `Dockerfile` and `scripts/start-devcontainer.sh`, it's okay to work and don't care about those fields at all but it's painful when coming to update and maintain those.