Added workflow to build custom version of Ghost

refs 7913283c96

- you can supply a branch and a version to call it, and it'll produce a
  zip of the tarball you can use
- this should avoid the need for local builds, which are error prone

.github/workflows/custom-build.yml

@@ -0,0 +1,20 @@
+on:
+  workflow_dispatch:
+    inputs:
+      branch:
+        description: 'Branch to build from'
+        type: string
+        required: false
+        default: 'main'
+      version:
+        description: 'Version to build'
+        type: string
+        required: true
+env:
+  FORCE_COLOR: 1
+jobs:
+  custom:
+    uses: tryghost/actions/.github/workflows/custom.yml@main
+    with:
+      branch: ${{ inputs.branch }}
+      version: ${{ inputs.version }}