Update versions for Zulip Server 5.7.

Dockerfile

@@ -28,7 +28,7 @@ WORKDIR /home/zulip
 # You can specify these in docker-compose.yml or with
 #   docker build --build-arg "ZULIP_GIT_REF=git_branch_name" .
 ARG ZULIP_GIT_URL=https://github.com/zulip/zulip.git
-ARG ZULIP_GIT_REF=5.6
+ARG ZULIP_GIT_REF=5.7
 
 RUN git clone "$ZULIP_GIT_URL" && \
     cd zulip && \

README.md

@@ -6,10 +6,10 @@ This is a container image for running [Zulip](https://zulip.com)
 ([GitHub](https://github.com/zulip/zulip)) in
 [production][prod-overview]. Image available from:
 
-* [**Docker Hub**](https://hub.docker.com/r/zulip/docker-zulip) (`docker pull zulip/docker-zulip:5.6-0`)
+* [**Docker Hub**](https://hub.docker.com/r/zulip/docker-zulip) (`docker pull zulip/docker-zulip:5.7-0`)
 
-Current Zulip version: `5.6`
-Current Docker image version: `5.6-0`
+Current Zulip version: `5.7`
+Current Docker image version: `5.7-0`
 
 Project status: **Alpha**.  While this project works and is
 used by many sites in production, configuring is substantially more

docker-compose.yml

@@ -47,13 +47,13 @@ services:
     volumes:
       - "/opt/docker/zulip/redis:/data:rw"
   zulip:
-    image: "zulip/docker-zulip:5.6-0"
+    image: "zulip/docker-zulip:5.7-0"
     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: "5.6"
+        ZULIP_GIT_REF: "5.7"
         # Set this up if you plan to use your own CA certificate bundle for building
         # CUSTOM_CA_CERTIFICATES:
     ports: