build: add docker action (#154)

## Description

it'll publish ghcr.io/milkyway-labs/milkywayd docker image to the GitHub
container registry

Closes: #XXXX

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files that
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---

### Author Checklist

*All items are required. Please add a note to the item if the item is
not applicable and
please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type
prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json)
in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR
Targeting](https://github.com/milkyway-labs/milkyway/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building
modules](https://docs.cosmos.network/v0.44/building-modules/intro.html)
- [ ] included the necessary unit and integration
[tests](https://github.com/milkyway-labs/milkyway/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go
code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable
and please add
your handle next to the items reviewed if you only reviewed selected
items.*

I have...

- [ ] confirmed the correct [type
prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json)
in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

.github/workflows/docker.yml

@@ -0,0 +1,68 @@
+name: Docker
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - "main"
+    tags:
+      - "v*"
+    paths:
+      - "**.go"
+      - "go.mod"
+      - "go.sum"
+  pull_request:
+    branches:
+      - "main"
+    paths:
+      - "**.go"
+      - "go.mod"
+      - "go.sum"
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: milkywayd
+
+jobs:
+  milkywayd:
+    name: Milkywayd
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    outputs:
+      tags: ${{ steps.meta.outputs.tags }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Create the builder
+        uses: docker/build-push-action@v4
+        with:
+          context: .
+          file: ./contrib/images/milkyway-builder/Dockerfile
+          load: true
+          tags: milkywaylabs/builder:latest
+
+      - name: Log in to the Container registry
+        uses: docker/login-action@v2
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@v4
+        with:
+          images: ${{ env.REGISTRY }}/${{ github.repository_owner }}/${{ env.IMAGE_NAME }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v4
+        with:
+          file: Dockerfile
+          push: ${{ startsWith(github.ref, 'refs/tags') }} # push image only for tags
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}