Set platform in docker compose

[cristianstanescu]

Trying to use docker compose on an M1 caused this error:

=> [internal] load build definition from Dockerfile                                                                                          0.0s
 => => transferring dockerfile: 32B                                                                                                           0.0s
 => [internal] load .dockerignore                                                                                                             0.0s
 => => transferring context: 2B                                                                                                               0.0s
 => ERROR [internal] load metadata for docker.io/library/eclipse-temurin:11-jdk-alpine                                                        0.6s
------
 > [internal] load metadata for docker.io/library/eclipse-temurin:11-jdk-alpine:
------
failed to solve: rpc error: code = Unknown desc = failed to solve with frontend dockerfile.v0: failed to create LLB definition: no match for platform in manifest sha256:8368526729a4fa4275807761f51770c505a16c8dcad969609d7644de9dab9367: not found

This PR fixes the issue by setting the platform parameter.

Furthermore, add a variable in the docker compose config to allow overriding the GraphDB version to install. I used the same name as in the preload config for consistency.