Switch Docker image source from Docker Hub to GHCR

- search/replace most image references
  - we're fortunate that hadolint is mirrored both places already
- replace direct upstream reference for "latest" `golang` image
  with our "mirror" image which is a very lightly touched
  variation of that image (mostly metadata tweaks)

refs GH-210

.github/workflows/lint-and-build-code.yml

@@ -35,7 +35,7 @@ jobs:
             experimental: true
 
     container:
-      image: "index.docker.io/atc0005/go-ci:${{ matrix.container-image}}"
+      image: "ghcr.io/atc0005/go-ci:${{ matrix.container-image}}"
 
     steps:
       - name: Check out code
@@ -66,7 +66,7 @@ jobs:
         container-image: ["go-ci-oldstable", "go-ci-stable", "go-ci-unstable"]
 
     container:
-      image: "index.docker.io/atc0005/go-ci:${{ matrix.container-image}}"
+      image: "ghcr.io/atc0005/go-ci:${{ matrix.container-image}}"
 
     steps:
       - name: Check out code
@@ -85,7 +85,7 @@ jobs:
         container-image: ["go-ci-oldstable", "go-ci-stable", "go-ci-unstable"]
 
     container:
-      image: "index.docker.io/atc0005/go-ci:${{ matrix.container-image}}"
+      image: "ghcr.io/atc0005/go-ci:${{ matrix.container-image}}"
 
     steps:
       - name: Print go version

.github/workflows/lint-and-build-using-make.yml

@@ -22,7 +22,8 @@ jobs:
     # Default: 360 minutes
     timeout-minutes: 10
     container:
-      image: "index.docker.io/golang:latest"
+      # Use (lightly touched) mirror of current "vanilla" upstream golang image
+      image: "ghcr.io/atc0005/go-ci:go-ci-stable-mirror-build"
 
     steps:
       - name: Print go version
@@ -58,7 +59,8 @@ jobs:
     # Default: 360 minutes
     timeout-minutes: 10
     container:
-      image: "index.docker.io/golang:latest"
+      # Use (lightly touched) mirror of current "vanilla" upstream golang image
+      image: "ghcr.io/atc0005/go-ci:go-ci-stable-mirror-build"
 
     steps:
       - name: Print go version

.github/workflows/lint-and-test-only.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     container:
-      image: index.docker.io/atc0005/go-ci:go-ci-lint-only
+      image: ghcr.io/atc0005/go-ci:go-ci-lint-only
 
     steps:
       - name: Check out code

.github/workflows/lint-docker-files.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     container:
-      image: index.docker.io/hadolint/hadolint:latest-debian
+      image: ghcr.io/hadolint/hadolint:latest-debian
 
     steps:
       - name: Check out code