Add ffmpeg package for the runner container (#4145)

* Revert "Update Dockerfile (#4090)"

This reverts commit 6758b0f1.

* Docker: Add ffmpeg package for the runner container

Misskey 10.81.0 added thumbnail generation support.
However it did not work with Docker bacause ffmpeg has not been
installed in the runner container.

Dockerfile

@@ -12,7 +12,6 @@ RUN unlink /usr/bin/free
 RUN apk add --no-cache \
     autoconf \
     automake \
-    ffmpeg \
     file \
     g++ \
     gcc \
@@ -36,7 +35,9 @@ RUN node-gyp configure \
 
 FROM base AS runner
 
-RUN apk add --no-cache tini
+RUN apk add --no-cache \
+    ffmpeg \
+    tini
 ENTRYPOINT ["/sbin/tini", "--"]
 
 COPY --from=builder /misskey/node_modules ./node_modules