Reapply "docker: Merge Dockerfiles into one again"

This reverts commit 57cd3c0.

backend/Dockerfile

@@ -1,21 +1,45 @@
-FROM ghcr.io/flathub-infra/backend-builder-base:latest AS builder
+FROM ubuntu:latest AS builder
 
-COPY pyproject.toml poetry.lock /
+RUN apt-get update && apt-get install -y --no-install-recommends software-properties-common \
+    && add-apt-repository ppa:flatpak/stable
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    python3 python3-dev python3-pip python-is-python3 python3-venv \
+    build-essential libpq-dev libcairo2-dev libgirepository1.0-dev \
+    gir1.2-ostree-1.0 gir1.2-appstream-1.0 flatpak
+
+RUN python -m pip install --no-cache-dir --break-system-packages pipx && \
+    pipx install poetry && \
+    pipx inject poetry poetry-plugin-export
 
+COPY pyproject.toml poetry.lock /
 RUN /root/.local/bin/poetry export -f requirements.txt --output requirements.txt && \
     python -m venv /venv && \
     /venv/bin/python -m pip install --no-cache-dir -r requirements.txt
 
-FROM ghcr.io/flathub-infra/backend-base:latest
+FROM ubuntu:latest
 ARG SENTRY_RELEASE
-ENV PATH="/venv/bin:$PATH"
 
-EXPOSE 8000
+RUN apt-get update && apt-get install -y --no-install-recommends software-properties-common \
+    && add-apt-repository ppa:flatpak/stable
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    python3 python-is-python3 python3-venv ca-certificates libcairo2 libpq5 \
+    libgirepository-1.0-1 gir1.2-ostree-1.0 gir1.2-appstream-1.0 flatpak && \
+    apt-get clean && rm -rf /var/lib/apt/lists/*
+
+RUN flatpak --user remote-add flathub https://flathub.org/repo/flathub.flatpakrepo && \
+    flatpak --user remote-add flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
 
 COPY --from=builder /venv /venv
 COPY . /app
 WORKDIR /app
 
-ENV SENTRY_RELEASE="$SENTRY_RELEASE"
+ENV PATH="/venv/bin:$PATH" \
+    SENTRY_RELEASE="$SENTRY_RELEASE"
+
+EXPOSE 8000
 
 CMD ["/app/docker-entrypoint.sh"]