build: add a PHP environment

php/Dockerfile

@@ -0,0 +1,20 @@
+FROM php:8.1-cli-bullseye
+
+ENV PATH=$PATH:/app/vendor/bin
+
+COPY --from=composer /usr/bin/composer /usr/bin/composer
+
+WORKDIR /app
+
+RUN apt-get update -yqq \
+  && apt-get install -yqq git unzip \
+  && adduser --disabled-password app \
+  && chown app:app -R /app
+
+USER app
+
+COPY --chown=app:app composer.* ./
+
+RUN composer install
+
+COPY --chown=app:app . .