README, Dockerfile: Update to support version in footer

README.adoc

@@ -289,8 +289,11 @@ where all repositories are found.
 Dockerfiles are provided in the `docker/` directory.
 To build the image, run the following commands:
 
- # git clone https://github.com/bootlin/elixir
- # docker build -t elixir -f ./elixir/docker/Dockerfile ./elixir/
+ # git clone https://github.com/bootlin/elixir.git ./elixir
+ # docker build -t elixir --build-arg ELIXIR_VERSION=`git rev-parse --short HEAD` ./elixir/Dockerfile ./elixir
+
+ELIXIR_VER build argument is optional. Since .git directory is not copied into Docker image by default,
+the option is used to pass a version string to Elixir.
 
 You can then run the image using `docker run`.
 Here we mount a host directory as Elixir data:

docker/Dockerfile

@@ -52,4 +52,7 @@ RUN a2enmod rewrite
 
 EXPOSE 80
 
+ARG ELIXIR_VERSION
+ENV ELIXIR_VERSION=$ELIXIR_VERSION
+
 ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]