diff --git a/scripts/deps-docker/Dockerfile b/scripts/deps-docker/Dockerfile
index 20e86a9e..b5e84a44 100644
--- a/scripts/deps-docker/Dockerfile
+++ b/scripts/deps-docker/Dockerfile
@@ -24,7 +24,9 @@ ENV     ver_asmcrypto=c72492f4a66e17a0e5dd8ad7874de354f3ccdaa5 \
 # the scp url is regular latin from https://fonts.googleapis.com/css2?family=Source+Code+Pro&display=swap
 RUN     mkdir -p /z/dist/no-pk \
         && wget https://fonts.gstatic.com/s/sourcecodepro/v11/HI_SiYsKILxRpg3hIP6sJ7fM7PqlPevW.woff2 -O scp.woff2 \
-        && apk add cmake make g++ git bash npm patch wget tar pigz brotli gzip unzip python3 python3-dev py3-brotli \
+        && apk add \
+                bash brotli cmake make g++ git gzip lame npm patch pigz \
+                python3 python3-dev py3-brotli sox tar unzip wget \
         && rm -f /usr/lib/python3*/EXTERNALLY-MANAGED \
         && wget https://github.com/openpgpjs/asmcrypto.js/archive/$ver_asmcrypto.tar.gz -O asmcrypto.tgz \
         && wget https://github.com/markedjs/marked/archive/v$ver_marked.tar.gz -O marked.tgz \
@@ -58,6 +60,11 @@ RUN     mkdir -p /z/dist/no-pk \
         && tar -xf zopfli.tgz
 
 
+COPY    busy-mp3.sh /z/
+RUN     /z/busy-mp3.sh \
+        && mv -v /dev/shm/busy.mp3.gz /z/dist
+
+
 # build fonttools (which needs zopfli)
 RUN     tar -xf zopfli.tgz \
         && cd zopfli* \
diff --git a/scripts/deps-docker/busy-mp3.sh b/scripts/deps-docker/busy-mp3.sh
new file mode 100755
index 00000000..08bd55a5
--- /dev/null
+++ b/scripts/deps-docker/busy-mp3.sh
@@ -0,0 +1,60 @@
+#!/bin/bash
+set -e
+
+cat >/dev/null <<EOF
+a frame is 1152 samples
+1sec @ 48000 = 41.66 frames
+11 frames = 12672 samples = 0.264 sec
+22 frames = 25344 samples = 0.528 sec
+EOF
+
+fast=1
+fast=
+
+echo
+cd /dev/shm
+rm -f -- a.*.mp3 a.*.mp3.gz
+min=99999999
+
+for freq in 425; do  # {400..500}
+for vol in 24; do  # {10..30}
+for kbps in 32; do
+for fdur in 1124; do  # {800..1200}
+for fdu2 in 1042; do  # {800..1200}
+for ftyp in h; do  # q h t l p
+for ofs1 in 801; do  # {0..4096}
+for ofs2 in 0; do  # {0..4096}
+for ofs3 in 0; do  # {0..4096}
+for nores in --nores; do  # '' --nores
+
+f=a.b$kbps$nores-f$freq-v$vol-$ftyp$fdur-$fdu2-o$ofs1-$ofs2-$ofs3
+sox -r48000 -Dn -r48000 -b16 -c2 -t raw s1.pcm synth 25344s sin $freq vol 0.$vol fade $ftyp ${fdur}s 25344s ${fdu2}s
+sox -r48000 -Dn -r48000 -b16 -c2 -t raw s0.pcm synth 12672s sin $freq vol 0
+tail -c +$ofs1 s0.pcm >s0a.pcm
+tail -c +$ofs2 s0.pcm >s0b.pcm
+tail -c +$ofs3 s0.pcm >s0c.pcm
+cat s{0a,1,0,0b,1,0c}.pcm > a.pcm
+lame --silent -r -s 48 --bitwidth 16 --signed a.pcm -m j --resample 48 -b $kbps -q 0 $nores $f.mp3
+if [ $fast ]
+then gzip -c <$f.mp3 >$f.mp3.gz
+else pigz -c11 -I1 <$f.mp3 >$f.mp3.gz
+fi
+sz=$(wc -c <$f.mp3.gz)
+printf '\033[A%d %s\033[K\n' $sz $f
+[ $sz -le $((min+10)) ] && echo
+[ $sz -le $min ] && echo && min=$sz
+
+done;done;done;done;done;done;done;done;done;done
+true
+
+f=a.b32--nores-f425-v24-h1124-1042-o801-0-0.mp3
+[ $fast ] &&
+    pigz -c11 -I1 <$f >busy.mp3.gz ||
+    mv $f.gz busy.mp3.gz
+
+sz=$(wc -c <busy.mp3.gz)
+[ "$sz" -eq 707 ] &&
+    echo busy.mp3 built successfully ||
+    echo WARNING: unexpected size of busy.mp3
+
+rm -f a.*.mp3 a.*.mp3.gz