Packer: file provisioner with docker builder hangs downloading a "big" file

[kidd]

Using docker builder, File provisioner in download direction (haven't tried upload) hangs in 1.4.1 and 1.4.2. 1.4.0 and prior are fine. host system is an ubuntu 18.04.

Bigger files than 64k approx hang forever when downloading from a docker builder.

```tmplate.json
{
"builders": [
{
"type": "docker",
"image": "ubuntu",
"discard": true
}
],
"provisioners": [
{
"type": "shell",
"inline": ["dd if=/dev/zero of=/tmp/file.txt count=64 bs=1024"]
},
{
"type": "file",
"direction": "download",
"source": "/tmp/file.txt",
"destination": "file.txt"
}
]

}

Run with `PACKER_LOG=1 packer build template.json`


change count=64 to count=164 and the process will hang

The logs stop after 'Downloading file from container...'

==> docker: Pausing before the next provisioner . Press enter to continue.
==> docker: Downloading /tmp/file.txt => file.txt
2019/07/15 13:25:33 packer: 2019/07/15 13:25:33 Downloading file from container: 389779dddebdd8ad721bcaa75478aae0a8dcde862d55488274dc58699519786d:/tmp/file.txt
```

[SwampDragons]

Good news! I was able to reproduce with v1.4.1 and v1.4.2 but wasn't able to reproduce on master, so I believe this has been fixed with the merge of https://github.com/hashicorp/packer/pull/7814

A build from when that patch was created can be found here: https://circleci.com/gh/hashicorp/packer/6178#artifacts/containers/0

The fix will be out with v1.4.3

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