Update `devcontainer.json`

[SergioGasquez]

• Include settings and extensions in the new customization property
• Add the image property to download the Dockerfile instead of building it from source

[iot49]

This works for me:

{
  "name": "espressif-training",
  "build": {
    "dockerfile": "Dockerfile",
    "args": {
      "NIGHTLY_VERSION": "nightly-2023-02-28"
    }
  },
  "customizations": {
    "vscode": {
      "settings": {
        "editor.formatOnPaste": true,
        "editor.formatOnSave": true,
        "editor.formatOnSaveMode": "modifications",
        "editor.formatOnType": true,
        "lldb.executable": "/usr/bin/lldb",
        "files.watcherExclude": {
          "**/target/**": true
        },
        "rust-analyzer.checkOnSave.command": "clippy",
        "[rust]": {
          "editor.defaultFormatter": "rust-lang.rust-analyzer"
        }
      },
      "extensions": [
        "rust-lang.rust-analyzer",
        "tamasfe.even-better-toml",
        "vadimcn.vscode-lldb",
        "serayuzgur.crates",
        "mutantdino.resourcemonitor",
        "yzhang.markdown-all-in-one"
      ]
    }
  },
  "remoteUser": "esp",
  "workspaceMount": "source=${localWorkspaceFolder},target=/workspace,type=bind,consistency=cached",
  "workspaceFolder": "/workspace"
}