build: fix localstack version in docker-compose to 1.2

[halfwhole]

Problem

docker-compose.yml is using localstack:latest, which has recently bumped its latest version from v1.2.0 to v1.3.0. Apparently the local AWS bucket somehow does not work the same way in localstack v1.3.0, and hence file uploads and E2E tests are failing.

Solution

Fix the version of localstack to 1.2 in docker-compose.yml.

(If we want to upgrade to localstack v1.3.0 eventually, we'll have to figure out how to make local S3 work with it.)

Other notes:

• Integration tests are helpful for debugging!
• Try pinning down specific versions of images instead of using latest, because it makes the project vulnerable to package upgrades which may suddenly and inexplicably break things
• Curiously I still can't replicate this problem locally, despite killing all docker containers and restarting them - does anyone have any ideas why?