Yes, it is open source, and Docker containers are freely accessible too

https://github.com/hasura/graphql-engine

https://hub.docker.com/r/hasura/graphql-engine

I run v2.x images myself, not sure what v3 and DDN are, besides monetization efforts for the company*.

Also the migrations are just `up.sql` and `down.sql` files, there is nothing coupling you to a proprietary migration format, the value Hasura offers is generating them for you and tracking their application thru their CLI.