Customiable docker image

[rumblefrog]

Allow alpine image in the action's Dockerfile to be customizable. I'm being ratelimited by Docker registry, and would like to point it at a proxy instead.

/usr/bin/docker build -t e66065:c5b87eb9cabe4ea89e79351984c7eb38 -f "/home/runner/_work/_actions/CodelyTV/pr-size-labeler/v1.10.2/Dockerfile" "/home/runner/_work/_actions/CodelyTV/pr-size-labeler/v1.10.2"
  #0 building with "default" instance using docker driver
  
  #1 [internal] load build definition from Dockerfile
  #1 transferring dockerfile: 442B done
  #1 DONE 0.0s
  
  #2 [internal] load metadata for docker.io/library/alpine:[3](https://github.twdcgrid.net/QDIE/pcs-configurator/actions/runs/105924/job/256508?pr=218#step:2:3).15
  #2 ERROR: failed to copy: httpReadSeeker: failed open: unexpected status code https://registry-1.docker.io/v2/library/alpine/manifests/sha256:19b[4](https://github.twdcgrid.net/QDIE/pcs-configurator/actions/runs/105924/job/256508?pr=218#step:2:4)bcc4f60e99dd[5](https://github.twdcgrid.net/QDIE/pcs-configurator/actions/runs/105924/job/256508?pr=218#step:2:5)ebdca0cbce22c503bbcff197549d7e19dab4f22254dc864: 429 Too Many Requests - Server message: toomanyrequests: You have reached your unauthenticated pull rate limit. https://www.docker.com/increase-rate-limit
  ------
   > [internal] load metadata for docker.io/library/alpine:3.15:
  ------
  Dockerfile:1
  --------------------
     1 | >>> FROM alpine:3.15
     2 |     
     3 |     RUN apk add --no-cache bash curl jq wget
  --------------------
  ERROR: failed to solve: alpine:3.15: failed to resolve source metadata for docker.io/library/alpine:3.15: failed to copy: httpReadSeeker: failed open: unexpected status code https://registry-1.docker.io/v2/library/alpine/manifests/sha25[6](https://github.twdcgrid.net/QDIE/pcs-configurator/actions/runs/105924/job/256508?pr=218#step:2:6):19b4bcc4f60e99dd5ebdca0cbce22c503bbcff19[7](https://github.twdcgrid.net/QDIE/pcs-configurator/actions/runs/105924/job/256508?pr=218#step:2:7)549d7e19dab4f22254dc[8](https://github.twdcgrid.net/QDIE/pcs-configurator/actions/runs/105924/job/256508?pr=218#step:2:8)64: 42[9](https://github.twdcgrid.net/QDIE/pcs-configurator/actions/runs/105924/job/256508?pr=218#step:2:9) Too Many Requests - Server message: toomanyrequests: You have reached your unauthenticated pull rate limit. https://www.docker.com/increase-rate-limit

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[HerrDerb]

Absolutely would love this. We use our own proxy registry to be unaffected when docker (or currently cloudflare again) is down. Really would love this feature.

Add a option for a custom docker image and a small docs of the minimum requirements of the image (installed tools etc)