small changes for deployment with docker compose

[Pascal-Delange]

• new env variable "enable_gc_tracing" (requires application default credentials to run)
• use alpine instead of distroless image to have access to wget to use in docker-compose health checks
• better documentation of what env variables do
• drop "VerifySessionCookie" logic in auth middleware, that was no longer supposed to be used but messed up all our error logging related to token validation