Property right of AssignmentExpression expected node to be of a type ["Expression"] but instead got null

I have an Ember project and I'm struggling to `ember build -prod` this inside a `node:8` (docker) container.

Here's `ember build -prod` on my localhost:

> cleaning up...
> Built project successfully. Stored in "tmp/dist".

Here is `ember build -prod` in a Docker context:

> cleaning up...
> Build failed.
> Build Error (broccoli-persistent-filter:Babel > [Babel: app]) in app/node_modules/ajv/lib/ajv.js
>
> app/node_modules/ajv/lib/ajv.js: Property right of AssignmentExpression expected node to be of a type ["Expression"] but instead got null

Not sure what this means, but

```
$ npm ls ajv
app@2.0.0 /var/www/app-ember
├─┬ ember-cli-eslint@4.2.3
│ └─┬ broccoli-lint-eslint@4.2.1
│ └─┬ eslint@4.19.1
│ ├── ajv@5.5.2
│ └─┬ table@4.0.2
│ └── ajv@5.5.2 deduped
└─┬ ember-cli-fastboot@2.0.4
└─┬ fastboot-express-middleware@1.2.0
└─┬ request@2.88.0
└─┬ har-validator@5.1.3
└── ajv@6.10.0
```

Ember is at 3.3, and I've tried multiple fastboot versions from 1.0 beta to 2.x. I was planning to update Ember but need to build a production Docker image before I do the dependency dance. Either way I don't feel comfortable upgrading things before I can get this build to work.

Because why would it work on localhost and fail on the Docker context? I `npm ci` from the `package-lock.json` so packages should be identical. Apart from the above mention of `ember-cli-fastboot`, I haven't got a clue where to begin to research this.