Add "Dockerfile.*" to default file associations.

[dawonn-haval]

A common practice for naming dockerfiles is "Dockerfile.XYZ", but vscode treats "Dockerfile" only as a file extension.
https://stackoverflow.com/a/51205428/12757156

This means that syntax highlighting and 'file-tree icons' are not enabled on those files, but this can be remedied on a per-workspace basis by configuring this setting:

{
    "files.associations": {
        "Dockerfile.*": "dockerfile"
   }
}

It would be grand if this setting could be enabled by default.