Make version envvar optional -- breaks dev override

The mandatory variable check runs even if the string with the mandatory
variable check is overridden by another compose file. This seems like a
flaw in docker-compose's behavior -- overrides should come first, then
evaluation, right? I'm sure there's a good reason things are the way
they are.

doc/contributor/how-to/run-qgreenland.md

@@ -62,20 +62,23 @@ The source code looks at these envvars, if set:
 
 #### Mandatory Docker Compose envvars
 
-* `QGREENLAND_VERSION`: The `nsidc/qgreenland` docker image tag to use.
-
-
 #### Optional Docker Compose envvars
 
 Our source code expects to run in a container and has hard-coded path constants. We
 should move these envvars and defaults into the source code, but for now they're for
 configuring the compose stack to route directories on the host to the hard-coded
 container locations.
 
+* `QGREENLAND_VERSION`: The `nsidc/qgreenland` docker image tag to use. Defaults to
+  `latest`.
 * `QGREENLAND_DATA_WORKING_STORAGE`: defaults to `./data/working-storage`
 * `QGREENLAND_DATA_PRIVATE_ARCHIVE`: defaults to `./data/private-archive`
 * `QGREENLAND_DATA_LOGS`: defaults to `./data/logs`
 
+Visit our [storage architecture reference
+documentation](../reference/architecture/storage.md) to learn more about storage
+locations.
+
 
 ## How to start the service stack
 

docker-compose.yml

@@ -5,7 +5,7 @@ services:
   # Luigi runs as a service and must have jobs submitted to it
   # (`scripts/run.sh`)
   luigi:
-    image: "nsidc/qgreenland:${QGREENLAND_VERSION:?QGREENLAND_VERSION must be set}"
+    image: "nsidc/qgreenland:${QGREENLAND_VERSION:-latest}"
     container_name: "luigi"
     volumes:
       # Code