feat(app): add endpoint to verify docker image availability

[lorenzo-cavazzi]

/deploy #persist

This PR adds a new endpoint to verify the availability of any remote Docker image in a v2 registry GET /images?image_url=<registry-image_url>.

I believe importing the utils function directly in the route is not the best strategy, but I wasn't sure if it's worth it to make an Image Class just for that.
Feel free to adapt the PR in any way that makes sense 😁

fix #989

[RenkuBot]

You can access the deployment of this PR at https://renku-ci-nb-990.dev.renku.ch

[olevski]

@lorenzo-cavazzi this is great. Thanks for doing this. I will just add a marshmallow schema for the request too. We use a plugin that allows us to do this. And with this you can get a 422 response when the request is not valid "for free". After that will approve and merge.

[olevski]

A few additional changes here:

• the response body is always blank because the status codes are enough to indicate if the image exists (200) or it does not (404)
• relying on the status codes allows us to remove the schema definition
• added a simple integration test for the endpoint