[enh] Provide Python 3.9 container

Based on Buster-slim

.gitlab-ci.yml

@@ -9,6 +9,7 @@ test_build_image:
     - docker build 3.6
     - docker build 3.7
     - docker build 3.8
+    - docker build 3.9
   except:
     - tags
 
@@ -48,3 +49,8 @@ build_3_8:
   extends: .build_image
   variables:
     PYTHON_VERSION: "3.8"
+
+build_3_9:
+  extends: .build_image
+  variables:
+    PYTHON_VERSION: "3.9"

3.9/Dockerfile

+FROM python:3.9-slim-buster
+
+# Install libsodium
+RUN apt update && \
+apt install --yes libsodium23 make && \
+rm -rf /var/lib/apt/lists
+
+# Install Poetry
+RUN pip3 install poetry