Constrain Python to version 3.12.3

`s3cmd` version 2.4.0 with Python 3.12.4 on Alpine 3.20 fails with an
`AttributeError`.

Author: dalenewby

Dockerfile

@@ -1,4 +1,6 @@
-FROM python:3-alpine
+# With Python 3.12.4 on Alpine 3.20, s3cmd 2.4.0 fails with an AttributeError.
+# See ITSE-1440 for details.
+FROM python:3.12.3-alpine
 
 # Current version of s3cmd is in edge/testing repo
 RUN echo https://dl-cdn.alpinelinux.org/alpine/edge/testing >> /etc/apk/repositories