No offence but this seems overcomplicated to me. I know docker and nodejs etc are all the trend, or maybe I'm missing something that makes this necessary?

If it's just a matter of generating a random sprite, this could be a simple front-end script that given a random seed outputs a deterministic avatar. That seed could be based on a user-email etc and now you have a "service" without a back-end. The images are simple enough that it should be very efficient to generate and faster than a http request (provided it's written considerately)... or if it _must_ be done in the backend, you could generate a http image response on the fly based on a seed passed in the request header, again in one simple script.

[edit]

Sorry I feel bad now, but I genuinely think there is a better approach. I'm hoping you might be open to it. Here's a proof of concept that add avatars to HN based on the username:

(Just paste it in the console on this page and scroll to the top.)

  const avatar = (r=1, p=16) => {
   const c = document.createElement('canvas');
   const x = c.getContext('2d');
   c.width = 8*p;
   c.height = 8*p;
   for (let i=28; i--;) {
    // xorshift
    r ^= r <<  13;
    r ^= r >>> 17;
    r ^= r <<  5;
    // pixels
    const X=i&3, Y=i>>2;
    if (r>>>28 > X*X+Y) {
     x.fillRect(
      p*3.5 + p*X,
      p*0.5 + p*Y,
     p, p);
     x.fillRect(
      p*3.5 - p*X,
      p*0.5 + p*Y,
     p, p);
    }
   }
   return c;
  };
  // Your HN avatar ! :)
  const seed = [...me.innerText]
   .map(c => c.charCodeAt(0))
   .reduce((a, b) => a + b);
  const sprite = avatar(seed);
  document.body.prepend(sprite);