fix(ci): use v-prefixed major version tag for container image

Change the floating major tag from :1 to :v1 to match consumer repo
references (e.g. ghcr.io/devrail-dev/dev-toolchain:v1).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: matthew-on-git

.github/workflows/build.yml

@@ -100,7 +100,7 @@ jobs:
           images: ${{ env.IMAGE_NAME }}
           tags: |
             type=semver,pattern={{version}},value=${{ steps.tag.outputs.tag }}
-            type=semver,pattern={{major}},value=${{ steps.tag.outputs.tag }}
+            type=semver,pattern=v{{major}},value=${{ steps.tag.outputs.tag }}
           labels: |
             org.opencontainers.image.source=https://github.com/devrail-dev/dev-toolchain
             org.opencontainers.image.description=DevRail developer toolchain container