build-wine/README.md: add comment about needing recent docker version

related:
https://github.com/spesmilo/electrum/issues/6971
https://github.com/spesmilo/electrum/pull/6981

.travis.yml

@@ -51,7 +51,7 @@ jobs:
         - TARGET_OS=Windows
       services:
         - docker
-      before_install:
+      before_install:  # we need a newer version of docker; 20.10+ seems to work
         - sudo rm -rf /var/lib/apt/lists/*
         - curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
         - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

contrib/build-wine/README.md

@@ -17,6 +17,10 @@ folder.
     $ sudo apt-get install -y docker-ce
     ```
 
+    Note: older versions of Docker might not work well
+    (see [#6971](https://github.com/spesmilo/electrum/issues/6971)).
+    If having problems, try to upgrade to at least `docker 20.10`.
+
 2. Build image
 
     ```