fix(backup,ci): make databases.csv optional and upgrade Docker CLI in image

- Handle missing or empty databases.csv gracefully with warnings and empty DataFrame
- Add unit tests for robust databases.csv loading behavior
- Adjust seed tests to assert warnings across multiple print calls
- Replace Debian docker.io with docker-ce-cli to avoid Docker API version mismatch
- Install required build tools (curl, gnupg) for Docker repo setup

https://chatgpt.com/share/697e6d9d-6458-800f-9d12-1e337509be4e

Dockerfile

@@ -3,33 +3,35 @@ FROM python:3.11-slim
 
 WORKDIR /app
 
-# Runtime + build essentials:
-# - rsync: required for file backup/restore
-# - ca-certificates: TLS
-# - docker-cli: needed if you want to control the host Docker engine (via /var/run/docker.sock mount)
-# - make: to delegate install logic to Makefile
-#
-# Notes:
-# - On Debian slim, the docker client package is typically "docker.io".
-# - If you only want restore-without-docker, you can drop docker.io later.
+# Base deps for build/runtime + docker repo key
 RUN apt-get update && apt-get install -y --no-install-recommends \
     make \
     rsync \
     ca-certificates \
-    docker.io \
     bash \
+    curl \
+    gnupg \
   && rm -rf /var/lib/apt/lists/*
 
+# Install Docker CLI (docker-ce-cli) from Docker's official apt repo
+RUN bash -lc "set -euo pipefail \
+  && install -m 0755 -d /etc/apt/keyrings \
+  && curl -fsSL https://download.docker.com/linux/debian/gpg \
+     | gpg --dearmor -o /etc/apt/keyrings/docker.gpg \
+  && chmod a+r /etc/apt/keyrings/docker.gpg \
+  && . /etc/os-release \
+  && echo \"deb [arch=\$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/debian \${VERSION_CODENAME} stable\" \
+     > /etc/apt/sources.list.d/docker.list \
+  && apt-get update \
+  && apt-get install -y --no-install-recommends docker-ce-cli \
+  && rm -rf /var/lib/apt/lists/*"
+
 # Fail fast if docker client is missing
+RUN docker version || true
 RUN command -v docker
 
 COPY . .
-
-# All install decisions are handled by the Makefile.
 RUN make install
 
-# Sensible defaults (can be overridden at runtime)
 ENV PYTHONUNBUFFERED=1
-
-# Default: show CLI help
 CMD ["baudolo", "--help"]