[Bug]: `container build` under `/tmp` fails to copy directories

### I have done the following

- [x] I have searched the existing issues
- [ ] If possible, I've reproduced the issue using the 'main' branch of this project

### Steps to reproduce

```
mkdir -p /tmp/test
cd /tmp/test

mkdir -p srcdir
echo foo > srcdir/foo

cat > Dockerfile << 'EOF'
FROM docker.io/library/alpine:3.20

COPY srcdir /dstdir
RUN ls -al /dstdir
EOF

container build -t test -f ./Dockerfile --progress plain --no-cache .
```

`/dstdir/foo` file is missing.

### Problem description

File contents must be copied when `COPY srcdir /dstdir`.

### Environment

```markdown
- OS: macOS
- Xcode: 27.0
- Container: main
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Reproduce the failure with the provided /tmp/test commands and inspect the container build path handling the Dockerfile COPY instruction. Trace how a directory source is packaged and copied, then verify completion by rebuilding and confirming that /dstdir/foo contains foo.

Written by the indexing model from the issue text.