Account, Contract API and unit tests

.github/CODEOWNERS

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+* @loongy @jazg

.github/workflows/build.yml

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+name: Docker Build
+on:
+  push:
+    branches:
+      - master
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Build and push
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          tags: renbot/multichain:latest
+          secrets: |
+            GIT_AUTH_TOKEN=${{ secrets.PERSONAL_ACCESS_TOKEN }}
+          build-args: |
+            GITHUB_TOKEN=${{ secrets.PERSONAL_ACCESS_TOKEN }}
+
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}