The image push pipeline buffered the entire Docker image tar 3x in
memory (export, tar wrap, Bytes copy), causing OOM kills for images
over ~1-2 GB. Replace the in-memory pipeline with a temp-file +
streaming upload: export to a NamedTempFile, then stream the outer
tar (header, 8 MiB file chunks, footer) directly into
upload_to_container via body_try_stream. Peak memory drops from ~3x
image size to ~8 MiB constant.
Also adds incremental export progress reporting every 100 MiB.