RUN deno task in Dockerfile

I have made a script to make DB migrations happen using "deno task migration", and I'd love for it to run that during deployment, using a Dockerfile. Currently my dockerfile looks like this:

FROM denoland/deno:1.37.1
WORKDIR /app
USER deno
EXPOSE 3001
RUN deno task migration init
RUN deno task migration run
CMD ["deno", "run", "--allow-env", "--allow-read", "--allow-write", "--allow-net", "./src/app.ts"]

But during the deployment (using Fly.io), I get an error " > [3/4] RUN deno task migration init:
#0 0.242 error: No config file found"

I'm a bit lost on what to do next, because those tasks run well locally, the deno.json is there at the root of my project. It looks like this:

{
"tasks": {
"prod": "deno run --allow-env --allow-read --allow-net ./src/app.ts",
"dev": "deno run --allow-env --allow-read --allow-net --watch ./src/app.ts",
"migration": "deno run --allow-env --allow-read --allow-write --allow-net ./src/database/migrate.ts"
}
}

Can someone point me to the correct direction? Thanks.