Define baseimage with arguments required in multiple stages

Change versions, add common paths

Keep drupal path

Fix comment

docker/Dockerfile

@@ -1,7 +1,21 @@
-# Use the official Drupal 10 image to build GEOS PHP extension.
-FROM drupal:10.1 as php-dependencies
+# Use the official Drupal 10 as base image.
+FROM drupal:10.1 as baseimage
+
+# Define common paths.
+ENV FARMOS_PATH=/var/farmOS
+ENV DRUPAL_PATH=/opt/drupal
+ENV COMPOSER_HOME=${FARMOS_PATH}
+
+# Set the farmOS and composer project repository URLs and versions.
+ARG FARMOS_REPO=https://github.com/farmOS/farmOS.git
+ARG FARMOS_VERSION=3.x
+ARG PROJECT_VERSION=3.x
+ARG PROJECT_REPO=https://github.com/farmOS/composer-project.git
+
+##
+# Build PHP extensions, GEOS and bcmath.
+FROM baseimage as php-dependencies
 
-# Build and install the GEOS PHP extension.
 # See https://git.osgeo.org/gitea/geos/php-geos
 ARG PHP_GEOS_VERSION=e77d5a16abbf89a59d947d1fe49381a944762c9d
 ADD https://github.com/libgeos/php-geos/archive/${PHP_GEOS_VERSION}.tar.gz /opt/php-geos.tar.gz
@@ -17,14 +31,9 @@ RUN apt-get update && apt-get install -y libgeos-dev \
 # Install the BCMath PHP extension.
 RUN docker-php-ext-install bcmath
 
-# Inherit from the official Drupal 10 image.
-FROM drupal:10.1
-
-# Set the farmOS and composer project repository URLs and versions.
-ARG FARMOS_REPO=https://github.com/farmOS/farmOS.git
-ARG FARMOS_VERSION=3.x
-ARG PROJECT_REPO=https://github.com/farmOS/composer-project.git
-ARG PROJECT_VERSION=3.x
+##
+# Build final image.
+FROM baseimage
 
 # Set Apache ServerName directive globally to suppress AH00558 message.
 RUN echo "ServerName localhost" >> /etc/apache2/apache2.conf