docker: change zulip ENVs to ARGs for easier updating.

This way, only the compose file needs to be touched which is more in
line with other configuration options.

Dockerfile

@@ -5,8 +5,10 @@
 FROM ubuntu:xenial-20171114
 LABEL maintainer="Alexander Trost <galexrt@googlemail.com>"
 
-ENV ZULIP_GIT_URL="https://github.com/zulip/zulip.git" \
-    ZULIP_GIT_REF="master"
+# You can specify these in docker-compose.yml or with
+#   docker build --build-args "ZULIP_GIT_REF=git_branch_name" .
+ARG ZULIP_GIT_URL=https://github.com/zulip/zulip.git
+ARG ZULIP_GIT_REF=master
 
 # First, we setup working locales
 RUN echo 'APT::Install-Recommends 0;' >> /etc/apt/apt.conf.d/01norecommends && \

README.md

@@ -367,21 +367,27 @@ you can find using `docker ps`)
 
 ### Upgrading from a Git repository
 
-1. Edit `Dockerfile` in this directory to specify the Git repository
-and the commit to use as `ZULIP_GIT_URL` and `ZULIP_GIT_REF`
-(e.g. `master` or `1.8.1` or
-`445932cc8613c77ced023125248c8b966b3b7528`).
-
-2. Edit your `docker-compose.yml` to comment out the `image` line, e.g.:
+1. Edit `docker-compose.yml` to comment out the `image` line, and
+  specify the Git commit you'd like to build the zulip container from.
+  E.g.:
 
 ```
   zulip:
     # image: "quay.io/galexrt/zulip:1.8.1-0"
     build:
       context: .
+      args:
+        # Change these if you want to build zulip from a different repo/branch
+        ZULIP_GIT_URL: https://github.com/zulip/zulip.git
+        ZULIP_GIT_REF: master
 ```
 
-3. Run `docker-compose build zulip` to build a Zulip Docker image from
+You can set `ZULIP_GIT_URL` to any clone of the zulip/zulip git
+repository, and `ZULIP_GIT_REF` to be any ref name in that repository
+(e.g. `master` or `1.8.1` or
+`445932cc8613c77ced023125248c8b966b3b7528`).
+
+2. Run `docker-compose build zulip` to build a Zulip Docker image from
    the specified Git version.
 
 Then stop and restart the container as described in the previous section.

docker-compose.yml

@@ -28,6 +28,10 @@ services:
     image: "quay.io/galexrt/zulip:1.8.1-0"
     build:
       context: .
+      args:
+        # Change these if you want to build zulip from a different repo/branch
+        ZULIP_GIT_URL: https://github.com/zulip/zulip.git
+        ZULIP_GIT_REF: master
     ports:
       - "80:80"
       - "443:443"