Add support for customizing output tag name

[Firehed]

Currently, the final --tag is based on the repository and server-stage, plus the commit hash. When using GHA's matrix builds to dynamically set --build-args, the resulting outputs either get trampled or need to have a dymaically set repository with the matrix value, which a) isn't necessarily an ideal name and b) can reduce cache re-use across the parallel builds (e.g. with a shared step to download or build dependencies).

I'm thinking the ability to translate this:

name: Build

on:
  push:
    branches:
      - main

jobs:
  build:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        locale:
          - en-US
          - en-us
          - fr-ca

    steps:
      # ...
      - uses: firehed/multistage-docker-build-action@v1
        with:
          repository: ghcr.io/example/webserver-${{ matrix.locale }}
          stages: dependencies
          server-stage: server
          build-args: locale=${{ matrix.locale }}

with something more like this:

    # ...
    steps:
      # ...
      - uses: firehed/multistage-docker-build-action@v1
        with:
          repository: ghcr.io/example/webserver
          stages: dependencies
          server-stage: server
          build-args: locale=${{ matrix.locale }}
          # new value
          server-name: server-${{ matrix.locale }}

Resulting changes:

Before	After
ghcr.io/example/webserver-en-us/dependencies:{internal}	ghcr.io/example/webserver/dependencies:{internal}
ghcr.io/example/webserver-en-us/dependencies:{internal}	ghcr.io/example/webserver/dependencies:{internal}
ghcr.io/example/webserver-en-ca/dependencies:{internal}	ghcr.io/example/webserver/dependencies:{internal}
ghcr.io/example/webserver-fr-ca/dependencies:{internal}	ghcr.io/example/webserver/dependencies:{internal}
ghcr.io/example/webserver-en-us/server:abc123	ghcr.io/example/webserver/server-en-us:abc123
ghcr.io/example/webserver-en-ca/server:abc123	ghcr.io/example/webserver/server-en-ca:abc123
ghcr.io/example/webserver-fr-ca/server:abc123	ghcr.io/example/webserver/server-fr-ca:abc123

Of note: the dependencies stage collapses, resulting in it being sharable across the matrix. This may not always be the intended behavior; it's important to make it clear when this is appropriate.

This should similarly apply for the testenv-stage.