Add image parameters to compose.yaml

Compose files can be parameterized using Bash-style (*sigh*) syntax
([1]). We add variables `JANUS_AGGREGATOR_IMAGE`,
`JANUS_MIGRATOR_IMAGE`, `DIVVIUP_API_IMAGE` and
`DIVVIUP_API_MIGRATOR_IMAGE` to allow setting Docker image tags like so:

```sh
DIVVIUP_API_IMAGE=myrepository.dev/divviup_api:0.0.1 \
  JANUS_AGGREGATOR_IMAGE=myrepository.dev/janus_aggregator:0.7.8 \
   docker compose up
```

These variables are interpolated into the `image` field of the `service`
elements ([2]). Because we also have a `build` element on the
`divviup-api` services, the behavior is to pull the specified image and
fall back to the `build` element if it can't be found ([3]). So if you
want to build `divviup-api` from source and use that, do:

```sh
DIVVIUP_API_IMAGE=divviup_api:1 docker compose up
```

`divviup_api:1` will be built from the local context, unless it's
already in the local Docker repository, and then launched.

We provide defaults for all these images that pull from Divvi Up owned
public artifact repositories. It's somewhat unfortunate that we use a
`divviup-api` version that is behind `main`, but on the other hand the
objective is to enable a demo experience, so it makes sense to pin
Janus and divviup-api versions where the demo is known to work
end-to-end.

[1]: https://docs.docker.com/compose/compose-file/12-interpolation/
[2]: https://docs.docker.com/compose/compose-file/05-services/#image
[3]: https://docs.docker.com/compose/compose-file/build/#using-build-and-image

README.md

@@ -48,6 +48,22 @@ This will get you up and running quickly for development purposes.
 `docker compose` will automatically reload containers when you make changes. Data is persisted
 until you `docker compose rm --volumes`.
 
+If you want to use image versions besides the defaults, you can use environment variables
+`JANUS_AGGREGATOR_IMAGE`, `JANUS_MIGRATOR_IMAGE`, `DIVVIUP_API_IMAGE` and
+`DIVVIUP_API_MIGRATOR_IMAGE` when invoking `docker compose`. If the tag set for `DIVVIUP_API_IMAGE`
+or `DIVVIUP_API_MIGRATOR_IMAGE` cannot be found, it will be built from local context. For example:
+
+```bash
+DIVVIUP_API_IMAGE=divviup_api:localversion \
+  JANUS_IMAGE=us-west2-docker.pkg.dev/divviup-artifacts-public/janus/janus_aggregator:0.7.18 \
+  docker compose up
+```
+
+If `divviup_api:localversion` is present in the local Docker repository, it will be used, but it
+will be built otherwise, and then run.
+`us-west2-docker.pkg.dev/divviup-artifacts-public/janus/janus_aggregator:0.7.18` will be pulled and
+run.
+
 Two Janus aggregators will be created for you, but are not automatically paired to divviup-api.
 Their information is:
 1. Address: `http://janus_1_aggregator:8080/aggregator-api`, Token: `0000`
@@ -58,6 +74,7 @@ testing client, they are mapped to `localhost:9001` and `localhost:9002`, respec
 
 If using the divviup CLI, consider compiling with the `--features admin` option. Also, set these
 environment variables.
+
 ```bash
 # This token is intentionally blank, but you'll still need to set the variable.
 export DIVVIUP_TOKEN=

compose.yaml

@@ -1,7 +1,7 @@
 # Local development-only compose file. This is not suitable for production!
 
 x-janus-common: &janus_common
-  image: us-west2-docker.pkg.dev/divviup-artifacts-public/janus/janus_aggregator:0.7.7
+  image: ${JANUS_AGGREGATOR_IMAGE:-us-west2-docker.pkg.dev/divviup-artifacts-public/janus/janus_aggregator:0.7.7}
   restart: always
   healthcheck:
     test: ["CMD", "/bin/sh", "-c", "wget http://0.0.0.0:8000/healthz -O - >/dev/null"]
@@ -12,7 +12,7 @@ x-janus-common: &janus_common
       condition: service_completed_successfully
 
 x-janus-migrate: &janus_migrate
-  image: us-west2-docker.pkg.dev/divviup-artifacts-public/janus/janus_db_migrator:0.7.7
+  image: ${JANUS_MIGRATOR_IMAGE:-us-west2-docker.pkg.dev/divviup-artifacts-public/janus/janus_db_migrator:0.7.7}
   command:
     - migrate
     - run
@@ -43,6 +43,7 @@ services:
         target: /docker-entrypoint-initdb.d/postgres_init.sql
 
   divviup_api_migrate:
+    image: ${DIVVIUP_API_MIGRATOR_IMAGE:-us-west2-docker.pkg.dev/janus-artifacts/divviup-api/divviup_api:0.3.9}
     build:
       context: .
       args:
@@ -66,6 +67,7 @@ services:
             - README.md
 
   divviup_api:
+    image: ${DIVVIUP_API_IMAGE:-us-west2-docker.pkg.dev/janus-artifacts/divviup-api/divviup_api:0.3.9}
     build:
       context: .
       args: