feat(versioning): Add SENTRY_VERSION env var for sentry, snuba, relay (#509)

This is in preparation for the upcoming CalVer transition. Introduces a general `$SENTRY_VERSION` env variable, defaulting to `latest`.

.env

@@ -1,2 +1,3 @@
 COMPOSE_PROJECT_NAME=sentry_onpremise
 SENTRY_EVENT_RETENTION_DAYS=90
+SENTRY_VERSION=latest

docker-compose.yml

@@ -7,6 +7,7 @@ x-sentry-defaults: &sentry_defaults
     context: ./sentry
     args:
       - SENTRY_IMAGE
+      - SENTRY_VERSION
   image: sentry-onpremise-local
   depends_on:
     - redis
@@ -31,7 +32,7 @@ x-snuba-defaults: &snuba_defaults
     - redis
     - clickhouse
     - kafka
-  image: 'getsentry/snuba:latest'
+  image: 'getsentry/snuba:$SENTRY_VERSION'
   environment:
     SNUBA_SETTINGS: docker
     CLICKHOUSE_HOST: clickhouse
@@ -179,7 +180,7 @@ services:
       - relay
   relay:
     << : *restart_policy
-    image: "getsentry/relay:latest"
+    image: "getsentry/relay:$SENTRY_VERSION"
     volumes:
       - type: bind
         read_only: true

sentry/Dockerfile

@@ -1,5 +1,6 @@
+ARG SENTRY_VERSION=latest
 ARG SENTRY_IMAGE
-FROM ${SENTRY_IMAGE:-getsentry/sentry:latest}
+FROM ${SENTRY_IMAGE:-getsentry/sentry:$SENTRY_VERSION}
 
 COPY . /usr/src/sentry