fix(docker): unpin python3 minor; Ubuntu 26.04 ships 3.13 by default

The Dockerfile pinned `python3.12` but the default UBUNTU_TAG=26.04
ships python3.13 as the default `python3`, and `python3.12` is no
longer in resolute's repos. apt-get fails:

  E: Unable to locate package python3.12

The bundled installer.py is python_requires >= 3.10 and works on
any modern python3, so switched to the meta-package `python3`. The
image now tracks whatever default Python the chosen Ubuntu release
ships — no per-image-bump churn needed when Ubuntu rotates its
default Python.

Same fix is forward-compatible: UBUNTU_TAG=24.04 (LTS) gives 3.12,
26.04 gives 3.13, future releases will Just Work.

Author: imanimanyara

Dockerfile

@@ -41,15 +41,18 @@ ENV DEBIAN_FRONTEND=noninteractive \
     LC_ALL=C.UTF-8
 
 # Minimal package set: nginx (the web server) + supervisor (the process
-# manager) + python3.12 (for local bundle rebuilds + healthchecks) +
-# gosu (drops privileges in the entrypoint without forking).
+# manager) + python3 (for local bundle rebuilds + healthchecks; tracks
+# whatever the default Python is on the target Ubuntu release — the
+# bundled installer.py is python_requires >= 3.10 and works on any
+# python3 ≥ 3.10) + gosu (drops privileges in the entrypoint without
+# forking).
 RUN apt-get update \
  && apt-get install -y --no-install-recommends \
         ca-certificates \
         curl \
         gosu \
         nginx \
-        python3.12 \
+        python3 \
         python3-pip \
         supervisor \
         tini \