use ubuntu 20.04 for workflows

.github/workflows/docker.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
 
     steps:

.github/workflows/go-test.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v1
@@ -21,4 +21,3 @@ jobs:
 
     - name: test
       run: go test ./...
-

.github/workflows/integration-test.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
 
     steps:

.github/workflows/jshint.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/release.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v1