add bullseye to automated tests

.github/workflows/bullseye/Dockerfile

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+FROM debian:bullseye
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Install the minimum amount of packages required for testing, which currently means:
+# - minimal set of packages required to run Python 3
+# - shipping versions of uWSGI and nginx (so that config files are put in the right places)
+# Also, make sure we have a sane default locale inside the container
+
+RUN apt-get update \
+ && apt-get dist-upgrade -y \
+ && apt-get install -y --no-install-recommends \
+    apt-utils \
+	ca-certificates \
+	locales \
+	curl \
+	tzdata \
+    git \
+    build-essential \ 
+    git \
+    incron \
+    nginx \
+    python3-pip \
+    python3-click \
+    python3-virtualenv \
+    uwsgi \
+    uwsgi-plugin-asyncio-python3 \
+    uwsgi-plugin-python3 \
+&& locale-gen en_US.UTF-8
+
+ENV LC_ALL=en_US.UTF-8
+ENV LANG=en_US.UTF-8
+
+VOLUME ["/run"]
+
+CMD ["/usr/bin/python3", "/run/piku.py"]