[Docker] use pip legacy resolver

Dockerfile

+FROM python:3.9-buster AS core
+
+# Configure Python to be nice inside Docker and pip to stfu
+ENV PYTHONUNBUFFERED 1
+ENV PYTHONDONTWRITEBYTECODE 1
+ENV PIP_DEFAULT_TIMEOUT 100
+ENV PIP_DISABLE_PIP_VERSION_CHECK 1
+ENV PIP_NO_CACHE_DIR 1
+ENV PIP_EXTRA_INDEX_URL https://edugit.org/api/v4/projects/461/packages/pypi/simple
+ENV PIP_USE_DEPRECATED legacy-resolver
+
+# Configure app settings for build and runtime
+ENV ALEKSIS_static__root /usr/share/aleksis/static
+ENV ALEKSIS_media__root /var/lib/aleksis/media
+ENV ALEKSIS_backup__location /var/lib/aleksis/backups
+
+# Install necessary Debian and PyPI packages for build and runtime
+RUN apt-get -y update && \
+    apt-get -y install eatmydata && \
+    eatmydata apt-get -y upgrade && \
+    eatmydata apt-get install -y --no-install-recommends \
+        build-essential \
+	gettext \
+	libpq5 \
+	libpq-dev \
+	libssl-dev \
+	netcat-openbsd \
+	yarnpkg && \
+    eatmydata pip install gunicorn django-compressor
+
+# Install extra dependencies
+ARG EXTRAS="ldap"
+RUN   case ",$EXTRAS," in \
+        (*",ldap,"*) \
+          eatmydata apt-get install -y --no-install-recommends \
+            libldap2-dev \
+            libsasl2-dev \
+            ldap-utils \
+	  ;; \
+      esac
+
+# Install core
+RUN set -e; \
+    mkdir -p /var/lib/aleksis/media /usr/share/aleksis/static /var/lib/aleksis/backups; \
+    eatmydata pip install AlekSIS-Core\[$EXTRAS\]
+
+# Declare a persistent volume for all data
+VOLUME /var/lib/aleksis
+
+# Define entrypoint and gunicorn running on port 8000
+EXPOSE 8000
+COPY docker-entrypoint.sh /usr/local/bin/entrypoint.sh
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]
+
+# Install assets
+FROM core as assets
+RUN eatmydata aleksis-admin yarn install
+
+# Clean up build dependencies
+FROM assets AS clean
+RUN set -e; \
+    eatmydata apt-get remove --purge -y \
+        build-essential \
+        gettext \
+        libpq-dev \
+        libssl-dev \
+        libldap2-dev \
+        libsasl2-dev \
+        yarnpkg; \
+    eatmydata apt-get autoremove --purge -y; \
+    apt-get clean -y; \
+    rm -f /var/lib/apt/lists/*_*; \
+    rm -rf /root/.cache