Use always latest image to build the Docker image

contrib/Dockerfile

@@ -1,17 +1,15 @@
-FROM readthedocs/build:4.0rc1
+FROM readthedocs/build:latest
 
 ARG username
-ARG uid=1000
+ARG uid=$uid
 ARG gid=$uid
-ARG label=latest
 
-ENV USERNAME ${username}
 ENV UID ${uid}
 ENV GID ${gid}
 
 USER root
-RUN groupadd --gid $GID $USERNAME
-RUN useradd -m --uid $UID --gid $GID $USERNAME
-USER $username
+RUN groupadd --gid ${GID} docscustom
+RUN usermod --uid ${UID} --gid ${GID} docs
+USER docs
 
 CMD ["/bin/bash"]

contrib/docker_build.sh

@@ -4,13 +4,8 @@ username=`id -nu`
 uid=`id -u`
 gid=`id -g`
 
-version=$1
-[ -n "${version}" ] || version="latest"
-
 docker build \
-    -t readthedocs/build:dev \
-    --build-arg username=${username} \
+    -t readthedocs/build:latest-dev \
     --build-arg uid=${uid} \
     --build-arg gid=${gid} \
-    --build-arg label=${version} \
     .

docs/development/buildenvironments.rst

@@ -83,6 +83,7 @@ DOCKER_VERSION
 
     Default: :djangosetting:`DOCKER_VERSION`
 
+
 Local development
 -----------------
 
@@ -93,12 +94,7 @@ case, it's necessary to make a new image that overrides this user::
     cd contrib/
     ./docker_build.sh
 
-You can also specify a specific image version to use::
-
-    cd contrib/
-    ./docker_build.sh 4.0rc1
-
-This will create a new image, ``readthedocs/build:dev``, which you can specify
-as the default image in local settings overrides::
+This will create a new image, ``readthedocs/build:latest-dev``,
+which you can re-tag as ``latest`` to make Read the Docs use this image instead::
 
-    DOCKER_IMAGE = 'readthedocs/build:dev'
+  docker tag readthedocs/build:latest-dev readthedocs/build:latest