This change allows one to override any Sentry service image, mostly for testing purposes. It also removes the SENTRY_VERSION variable as docker-compose makes it very hard to cascade default values for these. Next step is to have integration tests in getsentry/snuba and getsentry/relay (and possibly for getsentry/symbolicator) for getsentry/onpremise using this PR.
Also related: #596.
This follows the best-practice of mounting the config folder as a volume and removes the need to rebuild sentry images for config changes. Partially addresses #314.
