fix: use distroless base image to reduce size and attack surface

Switch from python:3.13-slim-trixie to distroless/python3-debian13
for the sidecar container. The virtualenv approach now extracts
runtime libraries from Debian packages using dpkg, eliminating the
need for apt and package management tools in the final image.

The image is 44% smaller (260MB vs 463MB on main, or 31% vs 377MB
from the previous commit) with 70% fewer packages (35 vs 115) while
maintaining zero HIGH/CRITICAL vulnerabilities. There is no shell
or package manager in the final image, reducing the attack surface
significantly.

Based on Google's distroless best practices.

Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>

containers/Dockerfile.sidecar

@@ -2,7 +2,7 @@
 # The container needs to provide and build two components:
 # * barman-cloud
 # * instance plugin
-# Both components are built before going into the final container
+# Both components are built before going into a distroless container
 
 # Build the manager binary
 FROM --platform=$BUILDPLATFORM golang:1.25.6 AS gobuilder
@@ -33,17 +33,24 @@ COPY ../internal/ internal/
 RUN --mount=type=cache,target=/go/pkg/mod --mount=type=cache,target=/root/.cache/go-build \
     CGO_ENABLED=0 GOOS=${TARGETOS:-linux} GOARCH=${TARGETARCH} go build -a -o manager cmd/manager/main.go
 
-# Build wheel files for Python dependencies
-FROM python:3.13-slim-trixie AS pythonbuilder
+# Build Python virtualenv with all dependencies
+# Using virtualenv ensures bytecode is compiled with correct timestamps
+FROM debian:trixie-slim AS pythonbuilder
 WORKDIR /build
 
+# Install postgresql-common and setup pgdg repository first
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends postgresql-common && \
+    /usr/share/postgresql-common/pgdg/apt.postgresql.org.sh -y
+
 # Install build dependencies
+# After pgdg repo setup, this ensures we get updated versions from apt.postgresql.org
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
-        postgresql-common \
-        build-essential && \
-    /usr/share/postgresql-common/pgdg/apt.postgresql.org.sh -y && \
-    apt-get install -y --no-install-recommends \
+        python3 \
+        python3-venv \
+        python3-dev \
+        build-essential \
         libpq-dev \
         liblz4-dev \
         libsnappy-dev
@@ -51,15 +58,43 @@ RUN apt-get update && \
 # Copy requirements
 COPY containers/sidecar-requirements.txt .
 
-# Build wheels with pip cache mount
-RUN --mount=type=cache,target=/root/.cache/pip \
-    pip wheel --wheel-dir=/wheels -r sidecar-requirements.txt
+# Create virtualenv and install dependencies
+# Compileall ensures all bytecode is freshly compiled with correct timestamps
+RUN python3 -m venv /venv && \
+    /venv/bin/pip install --upgrade pip setuptools wheel && \
+    /venv/bin/pip install --no-cache-dir -r sidecar-requirements.txt && \
+    /venv/bin/python -m compileall -q /venv
+
+# Download and extract runtime library packages
+# Using apt-get download + dpkg -x ensures we get all files from packages
+# Note: libcrypto is part of libssl3t64, and liblber is part of libldap2
+RUN mkdir -p /dependencies /build/downloads && \
+    cd /build/downloads && \
+    apt-get download \
+        libpq5 \
+        liblz4-1 \
+        libsnappy1v5 \
+        libssl3t64 \
+        libgssapi-krb5-2 \
+        libkrb5-3 \
+        libk5crypto3 \
+        libcom-err2 \
+        libkrb5support0 \
+        libldap2 \
+        libsasl2-2 \
+        libkeyutils1 \
+        libzstd1 && \
+    for deb in *.deb; do \
+        dpkg -x "$deb" /dependencies; \
+    done
 
 # Final sidecar image
-FROM python:3.13-slim-trixie
+# Using distroless base for minimal attack surface (no shell, no package manager)
+FROM gcr.io/distroless/python3-debian13:nonroot
 
 ENV SUMMARY="CloudNativePG Barman plugin" \
-    DESCRIPTION="Container image that provides the barman-cloud sidecar"
+    DESCRIPTION="Container image that provides the barman-cloud sidecar" \
+    PATH="/venv/bin:$PATH"
 
 LABEL summary="$SUMMARY" \
       description="$DESCRIPTION" \
@@ -71,23 +106,12 @@ LABEL summary="$SUMMARY" \
       version="" \
       release="1"
 
-# Install runtime dependencies
-RUN apt-get update && \
-    apt-get install -y --no-install-recommends \
-        postgresql-common && \
-    /usr/share/postgresql-common/pgdg/apt.postgresql.org.sh -y && \
-    apt-get install -y --no-install-recommends \
-        libpq5 \
-        liblz4-1 \
-        libsnappy1v5 && \
-    apt-get clean && \
-    rm -rf /var/lib/apt/lists/*
+# Copy virtualenv with pre-compiled bytecode
+COPY --from=pythonbuilder /venv /venv
 
-# Install wheels using bind mount (wheels not included in final layers)
-# and ensure all Python bytecode is freshly compiled with correct timestamps
-RUN --mount=type=bind,from=pythonbuilder,source=/wheels,target=/wheels \
-    pip install --no-cache-dir /wheels/*.whl && \
-    python -m compileall -q
+# Copy runtime libraries from extracted packages
+# All libraries are in /usr/lib/x86_64-linux-gnu
+COPY --from=pythonbuilder /dependencies/usr/lib /usr/lib
 
 # Copy Go manager binary
 COPY --from=gobuilder /workspace/manager /manager