fix(docker): install xz in the ffmpeg stage -- the golang image has none; found by building the image, which CI does not do on a pull request (second review)

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

Dockerfile

@@ -36,7 +36,11 @@ FROM golang:1.26 AS ffmpeg
 ARG FFMPEG_VERSION=7.0.2
 ARG FFMPEG_SHA256=abda8d77ce8309141f83ab8edf0596834087c52467f6badf376a6a2a4c87cf67
 
+# xz is not in the golang image (found by building it: `tar: xz: Cannot exec`).
 RUN set -eu; \
+    apt-get update; \
+    apt-get install -y --no-install-recommends xz-utils; \
+    rm -rf /var/lib/apt/lists/*; \
     mkdir -p /out; \
     for dir in releases old-releases; do \
       curl -fsSL -o /out/ffmpeg.tar.xz "https://johnvansickle.com/ffmpeg/${dir}/ffmpeg-${FFMPEG_VERSION}-amd64-static.tar.xz" && break; \