extract hard-coded default image builders into file

Signed-off-by: Avi Deitcher <avi@deitcher.net>

.github/workflows/ci.yml

@@ -26,10 +26,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.13
+    - name: Set up Go 1.16
       uses: actions/setup-go@v2
       with:
-        go-version: 1.13.4
+        go-version: 1.16.7
       id: go
 
     - name: Check out code
@@ -110,7 +110,7 @@ jobs:
         key: ${{ runner.os }}-linuxkit-${{ github.sha }}
         restore-keys: |
           ${{ runner.os }}-linuxkit-
-    
+
     - name: Build Packages
       run: |
         make -C pkg build
@@ -157,7 +157,7 @@ jobs:
         chmod ugo+x bin/linuxkit-amd64-linux
         sudo ln -s $(pwd)/bin/linuxkit-amd64-linux /usr/local/bin/linuxkit
         /usr/local/bin/linuxkit version
-    
+
     - name: Restore Package Cache
       uses: actions/cache@v2
       with:
@@ -165,7 +165,7 @@ jobs:
         key: ${{ runner.os }}-linuxkit-${{ github.sha }}
         restore-keys: |
           ${{ runner.os }}-linuxkit-
- 
+
     - name: Run Tests
       run: |
           cd test