[Docker Compatibility] Missing stdin/stdout streaming (-) in `finch cp`

[tinovyatkin]

Summary

Finch's cp command does not support using - (dash) for streaming tar archives via stdin/stdout. This feature is available in Docker CLI and is essential for efficient file transfer in development tooling.

Upstream Issue

This is an nerdctl limitation tracked at: containerd/nerdctl#4691

Docker Behavior

# Stream tar archive into container
cat archive.tar | docker cp - container:/path/

# Stream container contents to stdout
docker cp container:/var/log/app.log - | tar x -O | grep "ERROR"

Podman Compatibility

Podman fully supports stdin/stdout streaming with - in the cp command.

Impact

This gap affects tools that transfer files to/from containers:

• VS Code Docker extension (uses stdin streaming to write files efficiently)
• Backup/restore operations
• CI/CD pipelines injecting configs or extracting artifacts

Workaround

A workaround for VS Code Docker extension is implemented in PR #327 that:

• Uses shell commands with tar and mktemp for file operations
• Creates temporary files for write operations

# Read workaround
finch cp container:/path - | tar -xOf -

# Write workaround (uses temp file)
TMPFILE=$(mktemp) && cat > "$TMPFILE" && finch cp "$TMPFILE" container:/path && rm -f "$TMPFILE"

Suggested Action

Consider contributing the implementation upstream to nerdctl to maintain Docker CLI compatibility.

[pendo324]

Thank you for your detailed report @tinovyatkin

I'm tracking this in our backlog. We will work towards supporting this