Port forwarding does not work correctly in Codespaces when using `ghcr.io/rails/devcontainer/images/ruby`, but works with `mcr.microsoft.com/devcontainers/ruby`

[hamajyotan]

Describe the bug

When using GitHub Codespaces with a devcontainer based on ghcr.io/rails/devcontainer/images/ruby, the forwardPorts and portsAttributes configurations in .devcontainer/devcontainer.json do not behave as expected. Specifically, private port forwarding does not activate correctly upon container startup.
By contrast, when switching the base image to mcr.microsoft.com/devcontainers/ruby, the port forwarding works correctly and immediately after the Codespace starts.
This suggests an issue specific to the Rails-provided devcontainer image.

To Reproduce

Steps to reproduce the behavior:

1. Create a Codespace with a .devcontainer/devcontainer.json like this:

{
  "dockerFile": "Dockerfile",
  "forwardPorts": [3000],
  "portsAttributes": {
    "3000": {
      "label": "Rails Dev Server",
      "onAutoForward": "notify",
      "visibility": "private"
    }
  }
}

2. Use the following Dockerfile:

# RUBY_VERSION can be any compatible version
ARG RUBY_VERSION=3.4.3
FROM ghcr.io/rails/devcontainer/images/ruby:$RUBY_VERSION

3. Start the Codespace and run a web server (e.g., bin/rails server -b 0.0.0.0 -p 3000)

4. Try to access the forwarded port (3000) from the Codespaces UI.

5. You’ll see a connection failure (timeout).

6. Then, manually change the port visibility from "Private" → "Public" → "Private" via the UI.

7. Now, the port becomes accessible as expected.

Workaround

Changing the base image from:

FROM ghcr.io/rails/devcontainer/images/ruby:3.4.3

to:

FROM mcr.microsoft.com/devcontainers/ruby:3.4

resolves the issue. With this image, port forwarding works correctly at container startup.

Expected behavior

Port 3000 should be forwarded and accessible with the visibility set to private, immediately after Codespace startup, without requiring manual toggling.

Additional context

• This issue seems to occur only when using ghcr.io/rails/devcontainer/images/ruby.

• Adding a postStartCommand such as curl http://localhost:3000 does not help.

• Manually toggling the port visibility from Private → Public → Private always fixes the issue.

• The GitHub CLI workaround using gh codespace ports visibility also works, but it’s not ideal for long-term use.

• This issue reproduces not only when manually creating a .devcontainer, but also when using the rails new --devcontainer command to automatically generate the .devcontainer directory. In other words, even when following the official Rails workflow with the minimum setup, the port forwarding issue occurs.

[hamajyotan]

Just a quick follow-up: I’ve found that the issue can be avoided by removing the forwardPorts setting from the generated devcontainer.json.

Specifically, when using rails new --devcontainer, the generated file includes:

  "forwardPorts": [3000],

Commenting this out or deleting it seems to prevent the port-forwarding issue described earlier.

Also, I’m unsure whether this should ultimately be considered an issue for rails/devcontainer or for rails/rails, since the root cause appears to be related to the structure of the files generated by the --devcontainer option.

I hope this helps others encountering the same problem and contributes to identifying a more permanent solution.

[hamajyotan]

Closing this issue.

Initially, I suspected this might be specific to the ghcr.io/rails/devcontainer/images/ruby image, but after further investigation, I found that the same issue also occurs with the Microsoft-provided mcr.microsoft.com/devcontainers/ruby image.

Therefore, I believe this is not a problem specific to the Rails devcontainer.

I’ve documented the investigation and reproduction cases in the following GitHub Discussion comment. Please feel free to refer to it:

🔗 https://github.com/orgs/community/discussions/156546#discussioncomment-13035675