appveyor: explicitly set default build worker image

- also update paragraph on worker images to AppVeyor README

.appveyor.yml

@@ -48,6 +48,9 @@ skip_commits:
 #   build matrix configuration    #
 #---------------------------------#
 
+# Default build worker image
+image: Visual Studio 2015
+
 # Build Configurations: dll/static, regular/debug
 configuration:
   - dynamic

appveyor/.appveyor.yml.example-full

@@ -51,6 +51,9 @@ skip_commits:
 # are executed sequentially, each one taking 10-15 minutes.
 # Consider this when defining your build matrix. (A full matrix build takes more than 8 hours.)
 
+# Default build worker image
+image: Visual Studio 2015
+
 # Build Configurations: dll/static, regular/debug
 configuration:
   - dynamic

appveyor/.appveyor.yml.example-mini

@@ -19,6 +19,8 @@ skip_commits:
     - '**/*.md'
     - '.travis.yml'
 
+image: Visual Studio 2019
+
 # Build Configurations: dll/static, regular/debug
 configuration:
   - dynamic
@@ -33,10 +35,8 @@ environment:
   matrix:
   - CMP: vs2019
     BASE: 7.0
-    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
   - CMP: vs2019
     BASE: 3.15
-    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
 
 # Platform: processor architecture
 platform:

appveyor/README.md

@@ -53,3 +53,16 @@
 
  6. Push your changes and check
     [ci.appveyor.com](https://ci.appveyor.com/) for your build results.
+
+## Known Issues
+
+#### Build Worker Images
+The AppVeyor documentation on build worker images doesn't seem to fully
+describe the way things are handled internally.
+
+The tested and suggested reproducible way of defining the build worker image
+is shown in the example configuration files:
+
+ - Set the default image using the `image:` tag.
+ - Override the image for specific jobs by setting the
+   `APPVEYOR_BUILD_WORKER_IMAGE` environment variable.