chore(deps): update ubuntu docker tag to v26

Replace 'python' package (Python 2, removed in Ubuntu 24.04+)
with a simple 'ln -sf python3 python' symlink that works on
all Ubuntu versions.

.gitea/workflows/docker-build.yaml

@@ -3,6 +3,7 @@ name: Docker Build and Push
 on:
   pull_request:
   push:
+    branches: [master]
   schedule:
     - cron: '30 3 * * 3'
 

Dockerfile

@@ -1,12 +1,13 @@
-FROM ubuntu:18.04
+FROM ubuntu:26.04
 LABEL maintainer="Julien Cabillot <dockerimages@cabillot.eu>"
 
 # hadolint ignore=DL3008,SC3009,SC2086
 RUN export DEBIAN_FRONTEND=noninteractive && \
     export BUILD_PACKAGES=git-core && \
-    export RUNTIME_PACKAGES="python python3 bsdmainutils curl python3-six ca-certificates" && \
+    export RUNTIME_PACKAGES="python3 bsdmainutils curl python3-six ca-certificates" && \
     apt-get -qq update && \
     apt-get -qq --no-install-recommends --yes install ${BUILD_PACKAGES} ${RUNTIME_PACKAGES} && \
+    ln -sf /usr/bin/python3 /usr/bin/python && \
     git clone https://github.com/jvehent/cipherscan.git && \
     /cipherscan/cscan.sh -l && \
     apt-get -qq --yes remove --purge ${BUILD_PACKAGES} && \