ffmpeg randomly hangs

Running an `ffmpeg` operation on the default Alpine install yields peculiar results. `ffmpeg` will run for a bit then hard-hang completely, as in not responding to ^C, ^D, or ^Z at all (still shows up in tty). It also does this randomly; it can choose to hang at any point in the encoding process (which somehow includes the y/N overwrite prompt weirdly enough), though it is still possible to get through the encoding operation with some luck.

The operation that I ran was merging a video and audio file, low bitrate and 3 seconds long (video is silent): `ffmpeg -i reptile.mp4 -i reptile.m4a -c:v copy -c:a aac reptile.mp4`. It encoded successfully after about 15 tries.

Start by reproducing the reported command, `ffmpeg -i reptile.mp4 -i reptile.m4a -c:v copy -c:a aac reptile.mp4`, on the default Alpine install. Compare successful and hanging runs and collect relevant diagnostics; done means identifying the cause of the intermittent uninterruptible hang and confirming the operation completes reliably.

Written by the indexing model from the issue text.