Update docker.yml

Add cache for yarn and nextjs.
`yarn install` takes about 25s off
`yarn export` takes about 40s

.github/workflows/docker.yml

@@ -15,11 +15,35 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/setup-node@v3
-      - uses: actions/checkout@v3
+      - name: Setup
+        uses: actions/setup-node@v3
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Get yarn cache directory path
+        id: yarn-cache-dir-path
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+      - name: Yarn cache
+        uses: actions/cache@v3
+        id: yarn-cache # use this to check for `cache-hit` (`steps.yarn-cache.outputs.cache-hit != 'true'`)
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: ${{ runner.os }}-yarn-
+      - name: Nextjs cache
+        uses: actions/cache@v2
+        with:
+        # See here for caching with `yarn` https://github.com/actions/cache/blob/main/examples.md#node---yarn or you can leverage caching with actions/setup-node https://github.com/actions/setup-node
+          path: |
+            ~/.npm
+            ${{ github.workspace }}/.next/cache
+          # Generate a new cache whenever packages or source files change.
+          key: ${{ runner.os }}-nextjs-${{ hashFiles('**/package-lock.json') }}-${{ hashFiles('**.[jt]s', '**.[jt]sx') }}
+          # If source files changed but packages didn't, rebuild from a prior cache.
+          restore-keys: ${{ runner.os }}-nextjs-${{ hashFiles('**/package-lock.json') }}-
       - run: yarn install --frozen-lockfile
       - run: yarn export
-      - uses: actions/cache@v2
+      - name: Cache build output
+        uses: actions/cache@v2
         id: restore-build
         with:
           path: ./out/