work around an issue with docker/dockerhub (#10547)

* try to workaround an issue with docker/dockerhub
* apply workaround only when running in travis

share/spack/qa/run-docker-tests

@@ -21,9 +21,23 @@ ensure_docker_login() {
         return $__login_success
     fi
 
+    if [ "$CI" '=' 'true' -a "$TRAVIS" '=' 'true' ] ; then
+        # NOTE: work around an issue with docker/docker hub
+        # https://github.com/docker/hub-feedback/issues/1222
+        # https://github.com/docker/cli/issues/1180
+        rm -f $HOME/.docker/config.json
+    fi
+
     echo "$DOCKER_PASSWORD" | \
         docker login -u "$DOCKER_USERNAME" --password-stdin
 
+    if [ "$CI" '=' 'true' -a \
+         "$TRAVIS" '=' 'true' -a \
+         '!' -f "$HOME/.docker/config.json" ]
+    then
+        echo "Warning: config file $HOME/.docker/config.json not created" >&2
+    fi
+
     if [ $? '=' '0' ] ; then
           __login_success=0
     fi