ci: preinstall ffmpeg for live media checks

.github/images/live-media-runner/Dockerfile

@@ -0,0 +1,16 @@
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update \
+  && apt-get install -y --no-install-recommends \
+    bash \
+    ca-certificates \
+    curl \
+    ffmpeg \
+    git \
+    openssh-client \
+    unzip \
+    xz-utils \
+    zstd \
+  && rm -rf /var/lib/apt/lists/*