Add docker build file.

Dockerfile

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+# Inherit from the upsteam farmOS 3.x image.
+FROM farmos/farmos:3.x
+
+# Install `jq` to help in extracting the farmOS version below.
+RUN apt-get update && apt-get install -y jq
+
+# Create a fresh /var/farmOS directory.
+RUN rm -r /var/farmOS && mkdir /var/farmOS
+
+# Copy composer.json and composer.lock into the image.
+COPY composer.json /var/farmOS/composer.json
+COPY composer.lock /var/farmOS/composer.lock
+
+# Build the farmOS codebase with Composer as the www-data user in /var/farmOS
+# with the --no-dev flag.
+RUN (cd /var/farmOS; composer install --no-dev)
+
+# Set the version in farm.info.yml to match the version locked by Composer.
+# This is optional but is useful because the version will appear as the
+# "Installation Profile" version at `/admin/reports/status` in farmOS.
+RUN sed -i "s|version: 3.x|version: $(jq -r '.packages[] | select(.name == "farmos/farmos").version' /var/farmOS/composer.lock)|g" /var/farmOS/web/profiles/farm/farm.info.yml
+
+# Copy the farmOS codebase into /opt/drupal.
+RUN rm -r /opt/drupal && cp -rp /var/farmOS /opt/drupal