Add Dockerfile for PHP 8.4 environment setup

docker/Dockerfile

@@ -0,0 +1,22 @@
+# syntax=docker/dockerfile:1
+
+FROM serversideup/php:8.4-unit-bookworm AS sbs_base
+
+WORKDIR /
+VOLUME /var
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    acl \
+    file \
+    gettext \
+    git \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN set -eux; \
+    install-php-extensions \
+        @composer \
+        apcu \
+        intl \
+        opcache \
+        zip \
+    ;