Allow to run docker with DISABLE_LEDGER (#1430)

[sgillespie]

Description

Fixes #1430. This change allows users to pass an environment variable DISABLE_LEDGER to the docker container (and the NixOS module). If this flag is non-empty, it will pass --disable-ledger and omit the --state-dir ... option.

Note that even when services.cardano-db-sync.disableLedger is true, stateDir is still defaulted and required, because we write pgpass there.

Checklist

• Commit sequence broadly makes sense
• Commits have useful messages
• New tests are added if needed and existing tests are updated
• Any changes are noted in the changelog
• Code is formatted with fourmolu (which can be run with scripts/fourmolize.sh
• Self-reviewed the diff

Migrations

• The pr causes a breaking change of type a,b or c
• If there is a breaking change, the pr includes a database migration and/or a fix process for old values, so that upgrade is possible
• Resyncing and running the migrations provided will result in the same database semantically

If there is a breaking change, especially a big one, please add a justification here. Please elaborate
more what the migration achieves, what it cannot achieve or why a migration is not possible.

[andreabedini]

I understand you just want to check whether DISABLE_LEDGER is set or not but, given you write DISABLE_LEDGER=true in the documentation above, I am worried someone will do DISABLE_LEDGER=false 😂.
Maybe checking for an explicit value of DISABLE_LEDGER (e.g. "true" or 1) could be a safer option?

[sgillespie]

That's a good point. After looking around it, it looks like there's an existing pattern (RECREATE_DB=Y), so I think it would make sense to follow that pattern. Source: https://github.com/input-output-hk/cardano-db-sync/blob/master/scripts/postgresql-setup.sh#L311

[andreabedini]

Just a thought, could we get the networkMagic in nix rather than going through jq?