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

[phuctm97]

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.