ci(test): Fix 'pool is closed' errors (#826)

Fixes #823. In install.sh we build a local sentry image that is used by many services from using the build context under the ./sentry directory. To avoid building this image multiple times, we also give it a specific name which is referred from multiple services. The issue is, we also run docker-compose build --parallel which creates a race condition when building this image as docker-compose doesn't check whether the image is already there or not. This is the root cause of all these random failures: an unsurprising race condition.