Explictly fetch the whole Git history when building Docker image for PR

Otherwise, `actions/checkout@v2` will only fetch the latest commit,
leaving `git describe` unable to produce a useful version, cf.
actions/checkout#100 and
actions/checkout#217

Also switch to `actions/checkout@v2` when building the master branch and
release Docker images.

.github/workflows/push.yml

@@ -12,7 +12,11 @@ jobs:
     env:
       IMAGE: docker.io/${{ github.repository }}
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: "0"
+    - name: Fetch tags
+      run: git fetch --depth=1 origin +refs/tags/*:refs/tags/*
     - name: Set image version latest
       if: github.ref == 'refs/heads/master'
       run: echo ::set-env name=VERSION::latest

.github/workflows/test.yml

@@ -38,9 +38,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+      with:
+        fetch-depth: "0"
+    - name: Fetch tags
+      run: git fetch --depth=1 origin +refs/tags/*:refs/tags/*
     - name: Build image
-      env:
-        VERSION: v0.0.0+test
       run: make docker
   build-component-test:
     runs-on: ubuntu-latest