docker: Define ENTRYPOINT as absolute path

[110y]

What

• Use an absolute path for Dockerfile ENTRYPOINT

Why

I'm running up apollographql/router by Docker Compose for my local development. And, I'm changing the working directory for the router image like below in order to mount my application repository:

---
version: "3.8"

services:
  router:
    image: ghcr.io/apollographql/router:v1.0.0-alpha.0
    volumes:
      - .:/workspace:cached
    working_dir: /workspace
    command: --config router.yaml --supergraph schema.graphql

Since the current Dockerfile specifies a relative path (./router) for ENTRYPOINT, changing the working directory causes an error

Error response from daemon: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: exec: "./router": stat ./router: no such file or directory: unknown

Though I know specifying entrypoint: /dist/router to my docker-compose.yml fixes the problem, it requires users of the router image to recognize the working directory of the image (/dist) to do so, and I think it's not user friendly.
So, I created this PR that changes ENTRYPOINT from a relative ./router to an absolute /dist/router.

[apollo-cla]

@110y: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[abernix]

Ah, right — thanks! This was a result of me trying to DRY things up, but thank you.

[abernix]

Hope you don't mind, I added a change log entry for you!