Parsing config.yaml with environment variable substitution via B64 envvar in AWS ECS

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Hi. I'm trying to set up a trial/POC powersync implementation in AWS ECS and I am having trouble getting a powersync.yaml file passing to the task with substitutions for other environment variables. if I use the example straight from the website (https://github.com/powersync-ja/self-host-demo/blob/main/config/powersync.yaml) then powrsync complains with the following error:

{"level":"error","message":"Fatal error Could not collect config using Base64 method. Caught exception: Error: Could not parse PowerSync config file content as JSON or YAML: SyntaxError: Unexpected token '#', \"# yaml-lan\"... is not valid JSON","stack":"Error: Could not collect config using Base64 method. Caught exception: Error: Could not parse PowerSync config file content as JSON or YAML: SyntaxError: Unexpected token '#', \"# yaml-lan\"... is not valid JSON\n at CompoundConfigCollector.collectBaseConfig (file:///app/packages/service-core/dist/util/config/compound-config-collector.js:136:23)\n at async CompoundConfigCollector.collectConfig (file:///app/packages/service-core/dist/util/config/compound-config-collector.js:28:28)\n at async startUnifiedRunner (file:///app/service/lib/runners/unified-runner.js:11:20)\n at async Command.<anonymous> (file:///app/packages/service-core/dist/entry/commands/start-action.js:12:9)\n at async Command.parseAsync (/app/node_modules/.pnpm/[email protected]/node_modules/commander/lib/command.js:1092:5)\n at async runProgram (file:///app/packages/service-core/dist/entry/cli-entry.js:31:17)","timestamp":"2025-04-08T08:55:47.137Z"}

If I remove all of the !env entries in the config file then it appears to parse correctly but I obviously have an incomplete configuration. I have also tried using {envvar name} as placeholders (e.g. url: {PS_DATA_SOURCE_URL} and this also parses but fails with the following error message:

{"level":"error","message":"Fatal error Failed to validate Module Postgres configuration: /hostname: type must be string, /port: type must be number, /port: type must be string, /port: anyOf must match a schema in anyOf","stack":"Error: Failed to validate Module Postgres configuration: /hostname: type must be string, /port: type must be number, /port: type must be string, /port: anyOf must match a schema in anyOf\n at PostgresModule.validateConfig (file:///app/packages/service-core/dist/replication/ReplicationModule.js:59:19)\n at PostgresModule.decodeConfig (file:///app/packages/service-core/dist/replication/ReplicationModule.js:45:14)\n at PostgresModule.initialize (file:///app/packages/service-core/dist/replication/ReplicationModule.js:40:14)\n at PostgresModule.initialize (file:///app/modules/module-postgres/dist/module/PostgresModule.js:22:21)\n at ModuleManager.initialize (file:///app/packages/service-core/dist/modules/ModuleManager.js:20:26)\n at startUnifiedRunner (file:///app/service/lib/runners/unified-runner.js:20:25)\n at async Command.<anonymous> (file:///app/packages/service-core/dist/entry/commands/start-action.js:12:9)\n at async Command.parseAsync (/app/node_modules/.pnpm/[email protected]/node_modules/commander/lib/command.js:1092:5)\n at async runProgram (file:///app/packages/service-core/dist/entry/cli-entry.js:31:17)","timestamp":"2025-04-08T09:19:13.161Z"}

Am I doing something fundamentally wrong? Do substitutions not work, as suggested in the comments within the sample config file? Do I need to pre-build a config file pulling in values from AWS secrets where appropriate before passing to the ECS Task as a base 64 encoded config?

Any help would be much appreciated.