Build the container on Debian trixie for ffmpeg 7.1 (#177)

Bookworm's ffmpeg is 5.1.9, two major versions behind, and DeadAir runs
everything through it. Both stages move, since the binary built in the
first has to find the same glibc in the second.

Checked in the new image on techpod-358 with a 643-term drop expression,
the size that failed with a bsf_list error on 5.1 today: the copy is
352.73s shorter, as 643 cuts of 21 frames at 26.122ms predict, and
decodes without errors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- The container is built on Debian 13 (trixie) instead of 12, so DeadAir uses ffmpeg 7.1 instead of 5.1 (#177).
+
 ## [0.11.0] - 2026-10-09
 
 ### Added