fix: replace python2 with python3+ln symlink for Ubuntu 26.04

'python' (Python 2) package removed in Ubuntu >=24.04.
Use python3 + ln -sf symlink instead, same as cipherscan#7.

Dockerfile

@@ -6,9 +6,10 @@ LABEL maintainer="Julien Cabillot <dockerimages@cabillot.eu>"
 
 RUN export DEBIAN_FRONTEND="noninteractive" && \
     export BUILD_PACKAGES="wget xz-utils" && \
-    export RUNTIME_PACKAGES="python xvfb libfontconfig libxrender1 libxcomposite1" && \
+    export RUNTIME_PACKAGES="python3 xvfb libfontconfig1 libxrender1 libxcomposite1" && \
     apt-get -qq update && \
     apt-get -qq --yes install ${BUILD_PACKAGES} ${RUNTIME_PACKAGES} && \
+    ln -sf /usr/bin/python3 /usr/bin/python && \
     wget -nv -O- https://download.calibre-ebook.com/linux-installer.py | python -c "import sys; main=lambda:sys.stderr.write('Download failed\n'); exec(sys.stdin.read()); main()" && \
     apt-get -qq --yes remove --purge ${BUILD_PACKAGES} && \
     apt-get -qq --yes autoremove --purge && \