[IMP] Install testing requirements into separate image.

Fixes #277

15.0/Dockerfile

@@ -1,5 +1,5 @@
 FROM debian:bullseye-slim
-MAINTAINER Odoo S.A. <info@odoo.com>
+LABEL maintainer="Odoo S.A. <info@odoo.com>"
 
 SHELL ["/bin/bash", "-xo", "pipefail", "-c"]
 

15.0/test.Dockerfile

@@ -0,0 +1,12 @@
+ARG version=15
+ARG image=odoo:${version}
+FROM ${image}
+LABEL maintainer="Odoo S.A. <info@odoo.com>"
+
+# Install testing requirements
+USER root
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends chromium && \
+    rm -rf /var/lib/apt/lists/*
+RUN pip3 install websocket-client
+USER odoo