Go back to buster and safe the GPG according to https://docs.mopidy.com/en/latest/installation/debian/.

Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:bullseye-slim
+FROM debian:buster-slim
 
 RUN set -ex \
     # Official Mopidy install for Debian/Ubuntu along with some extensions
@@ -10,7 +10,7 @@ RUN set -ex \
         gnupg \
         gstreamer1.0-alsa \
         gstreamer1.0-plugins-bad \
-        python3-cryptography \
+        python3-crypto \
         python3-distutils \
  && curl -L https://bootstrap.pypa.io/get-pip.py | python3 - \
  && pip install pipenv \
@@ -19,7 +19,8 @@ RUN set -ex \
  && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* ~/.cache
 
 RUN set -ex \
- && curl -L https://apt.mopidy.com/mopidy.gpg | apt-key add - \
+ && mkdir -p /usr/local/share/keyrings \
+ && curl -L https://apt.mopidy.com/mopidy.gpg -o /usr/local/share/keyrings/mopidy-archive-keyring.gpg \
  && curl -L https://apt.mopidy.com/mopidy.list -o /etc/apt/sources.list.d/mopidy.list \
  && apt-get update \
  && DEBIAN_FRONTEND=noninteractive apt-get install -y \