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your

containerDefinitions

is a standard ts string, but the value of

ecr.repositoryUrl

is an unresolved promise. You need to build your

JSON.stringify

string inside the apply itself, so try this

const containerDefinitions = ecr.repositoryUrl.apply(url => JSON.stringify(
[
{
    name: this.FormattedName,
    image: pulumi.interpolate`${url}:${imageVersion}`,
    portMappings: portMappings,
    environment: environmentVariables
   }

])

That certainly has turned it into a proper output now. I suspect it'll be fine when I actually deploy it (though I'm not ready to do that in dev just yet). Thank you for the help!