You can install system packages into the deployed image using the aptGet extension: 
import { defineConfig } from "@trigger.dev/sdk/v3";
import { aptGet } from "@trigger.dev/build/extensions/core";

export default defineConfig({
  project: "<project ref>",
  // Your other config settings...
  build: {
    extensions: [aptGet({ packages: ["ffmpeg"] })],
  },
});
If you want to install a specific version of a package, you can specify the version like this: 
import { defineConfig } from "@trigger.dev/sdk/v3";

export default defineConfig({
  project: "<project ref>",
  // Your other config settings...
  build: {
    extensions: [aptGet({ packages: ["ffmpeg=6.0-4"] })],
  },
});