Bind GlitchTip version (and upgrade) to v4.0

Previously the `docker-compose.yml` we copied from GlitchTip's
instructions didn't specify a version tag. I don't know a ton about
Docker, but I imagine the deal here is that this means that we could
have used some Docker command like `docker pull` to update what version
of `glitchtip/glitchtip` is stored in our local image store or
something?

Anyway, I wanted to upgrade to version 4.0 (I'm not even actually sure
what version we were on before), and so the way I chose to do this was
to add a version specifier to the `docker-compose.yml` file. Then, all
we had to do was deploy it as usual, and Docker seemed to handle
downloading the image when starting up the service the next time, and
auto-performed the migrations, if I'm understanding correctly. (I did
see migrations being performed in the logs!) Great!

files/docker-compose.yml

@@ -28,7 +28,7 @@ services:
     image: redis
     restart: unless-stopped
   web:
-    image: glitchtip/glitchtip
+    image: glitchtip/glitchtip:v4.0
     depends_on: *default-depends_on
     ports:
       - "8000:8000"
@@ -38,7 +38,7 @@ services:
       - uploads:/code/uploads
     env_file: glitchtip-conf.env
   worker:
-    image: glitchtip/glitchtip
+    image: glitchtip/glitchtip:v4.0
     command: ./bin/run-celery-with-beat.sh
     depends_on: *default-depends_on
     environment: *default-environment
@@ -47,7 +47,7 @@ services:
       - uploads:/code/uploads
     env_file: glitchtip-conf.env
   migrate:
-    image: glitchtip/glitchtip
+    image: glitchtip/glitchtip:v4.0
     depends_on: *default-depends_on
     command: "./manage.py migrate"
     environment: *default-environment