fix: do not close caller-owned writer in container DefaultHandler

[officialasishkumar]

Summary

When pack runs a lifecycle phase, container.DefaultHandler copies the container's stdout/stderr into the phase writers and, once copying finishes, calls optionallyCloseWriter on each writer. That helper closed any writer that implemented io.Closer.

For untrusted builders the phase writers are *logging.PrefixWriter, whose Close only flushes a buffered partial line and leaves the wrapped writer open, so closing them is correct and necessary. For trusted builders, however, the logger's writer is passed through unwrapped. When a consumer builds the logger with os.Stdout (an *os.File, and therefore an io.Closer), running a build closed os.Stdout as a side effect.

The pack CLI never noticed because it exits immediately afterwards, but it is surprising and breaks programs that embed pack as a library and expect the writer they passed in to stay open (reported in #1214).

This change restricts optionallyCloseWriter to *logging.PrefixWriter so that only the buffering writers pack itself creates are flushed, and writers owned by the caller are left untouched.

Output

Using pack as a library with a trusted builder:

packClient, _ := pack.NewClient(pack.WithLogger(logging.NewSimpleLogger(os.Stdout)))
_ = packClient.Build(ctx, opts)
// continue using os.Stdout...

Before

os.Stdout is closed once Build returns, so any subsequent writes to it fail.

After

os.Stdout stays open after Build returns. Prefixed phase output for untrusted builders is still flushed exactly as before.

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

Resolves #1214