Hitting this (again) today. Note that this in your .devcontainer.json will cause the editor to open *.md files by default as previews:

"customizations": {
    "vscode": {
      "settings": {
        "workbench.editorAssociations": {
            "*.md": "vscode.markdown.preview.editor"
        }
    }
}

but this setting is not honored by the openFiles setting it seems.

(Oh and it causes diffs to be shown using previews, which is less than ideal.)

Same. I only want certain files to be opened automatically in preview mode.

Using the setting above I got this to work in conjunction with the post attach commands. The devcontainer.json has both:

"customizations": {
    "vscode": {
      "settings": {
        "workbench.editorAssociations": {
            "*.md": "vscode.markdown.preview.editor"
        }
    }
}

"postAttachCommand": "bash .devcontainer/post-attach.sh",

Then in post-attach.sh:

code docs/Codespaces.md

Using code respects the VSCode customization where OpenFiles configuration in the devcontainer.json does not.

Also if you only want the specific file to open in preview mode, you can also set *.md to the exact filename.

This does output to the terminal when it opens. To me it's worth the output.