Switch to DOCKER_TAG variable in order to make Docker Hub Build working

Dockerfile

 FROM debian:stretch
 ARG release
-ENV release=${release}
+ENV release=${DOCKER_TAG}
 RUN apt-get -y update && apt-get install -y \
     libx11-6 \
     libpng16-16 \

Dockerfile.dev

 FROM debian:stretch
 ARG release
-ENV release=${release}
+ENV release=${DOCKER_TAG}
 RUN apt-get -y update && apt-get install -y \
     libx11-dev \
     libpng-dev \

Dockerfile.multistage

 FROM debian:stretch as builder
 ARG release
-ENV release=${release}
+ENV release=${DOCKER_TAG}
 RUN apt-get -y update && apt-get install -y \
     libx11-dev \
     libpng-dev \
@@ -23,7 +23,7 @@ RUN curl http://www.iram.fr/~gildas/dist/gildas-src-$release.tar.xz | tar xJ &&
 
 FROM debian:stretch
 ARG release
-ENV release=${release}
+ENV release=${DOCKER_TAG}
 RUN apt-get -y update && apt-get install -y \
     libx11-6 \
     libpng16-16 \

README.md

@@ -5,15 +5,15 @@
 Build the developpement image :
 
 ```bash
-export release=mar18c
-docker build --tag abeelen/gildas:build  --build-arg release=$release -f Dockerfile.dev .
+export DOCKER_TAG=mar18c
+docker build --tag abeelen/gildas:build  --build-arg DOCKER_TAG=$DOCKER_TAG -f Dockerfile.dev .
 ```
 
 extract the compiled files
 
 ```
 docker container create --name extract abeelen/gildas:build
-docker container cp extract:/gildas-exe-$release gildas-exe-$release
+docker container cp extract:/gildas-exe-$DOCKER_TAG gildas-exe-$DOCKER_TAG
 docker container rm -f extract
 ```
 
@@ -22,13 +22,13 @@ extract the compiled files
  ```
  docker run -it -u $(id -u) \
      -v "/home/$USER:/home/$USER" \
-     abeelen/gildas:build tar cvzf $PWD/gildas-exe-$release.tar.gz /gildas-exe-$release
+     abeelen/gildas:build tar cvzf $PWD/gildas-exe-$DOCKER_TAG.tar.gz /gildas-exe-$DOCKER_TAG
  ```
 --->
 
 Build the execution image :
 ```bash
-docker build --tag abeelen/gildas:$release --tag abeelen/gildas:latest --build-arg release=$release -f Dockerfile .
+docker build --tag abeelen/gildas:$DOCKER_TAG --tag abeelen/gildas:latest --build-arg release=$DOCKER_TAG -f Dockerfile .
 ```
 
 ## One stage build
@@ -36,8 +36,8 @@ docker build --tag abeelen/gildas:$release --tag abeelen/gildas:latest --build-a
 With Docker 17.05 or higher :
 
 ```bash
-export release=mar18c
-docker build --tag abeelen/gildas:$release --tag abeelen/gildas:latest --build-arg release=$release -f Dockerfile.multistage .
+export DOCKER_TAG=mar18c
+docker build --tag abeelen/gildas:$DOCKER_TAG --tag abeelen/gildas:latest --build-arg release=$DOCKER_TAG -f Dockerfile.multistage .
 ```
 
 # Usage