Fix: Add Docker login to GHCR before pushing images (fixes #153) (#154)

## Summary
Add `docker/login-action` to authenticate with GitHub Container Registry
before GoReleaser attempts to push Docker images.

## Problem
Docker push was failing with authentication error:
```
unauthorized: unauthenticated: User cannot be authenticated 
with the token provided.
The push refers to repository [ghcr.io/bsubio/cli]
```

The `credentials:` in the workflow's `container:` section only
authenticates pulling the CI container image. It doesn't log Docker in
for pushing images from within the container.

## Solution
Add explicit Docker login step using `docker/login-action` before
GoReleaser runs. This authenticates Docker to push images to ghcr.io.

## Changes
- Add "Log in to GitHub Container Registry" step
- Use `docker/login-action@v3`
- Authenticate to `ghcr.io` registry
- Use `github.actor` and `GITHUB_TOKEN`
- Runs after buildx setup, before GoReleaser

## Workflow Flow
1. ✅ Set up QEMU (for ARM64 emulation)
2. ✅ Set up Docker Buildx (for multi-arch builds)
3. ✅ **Log in to GHCR** (NEW - authenticate for push)
4. ✅ Run GoReleaser (build and push images)

## Result
- GoReleaser can successfully push multi-arch Docker images to ghcr.io
- Both amd64 and arm64 images are built and pushed
- Multi-arch manifests are created
- Users can pull images: `docker pull ghcr.io/bsubio/cli:latest`

Fixes #153

Author: wkoszek

.github/workflows/build.yml

@@ -16,7 +16,7 @@ on:
 
 permissions:
   contents: write
-  packages: read
+  packages: write
 
 jobs:
   build:
@@ -74,6 +74,14 @@ jobs:
         if: github.ref == 'refs/heads/main' && github.event_name == 'push'
         uses: docker/setup-buildx-action@v3
 
+      - name: Log in to GitHub Container Registry
+        if: github.ref == 'refs/heads/main' && github.event_name == 'push'
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
       - name: Get latest tag
         if: github.ref == 'refs/heads/main' && github.event_name == 'push'
         id: get_tag