GUACAMOLE-271: add Duo to Docker build

guacamole-docker/bin/build-guacamole.sh

@@ -134,3 +134,18 @@ if [ -f extensions/guacamole-auth-openid/target/guacamole-auth-openid*.jar ]; th
     mkdir -p "$DESTINATION/openid"
     cp extensions/guacamole-auth-openid/target/guacamole-auth-openid*.jar "$DESTINATION/openid"
 fi
+
+#
+# Copy Duo auth extension if it was built
+#
+
+if [ -f extensions/guacamole-auth-duo/target/*.tar.gz ]; then
+    mkdir -p "$DESTINATION/duo"
+    tar -xzf extensions/guacamole-auth-duo/target/*.tar.gz \
+        -C "$DESTINATION/duo/"                             \
+        --wildcards                                        \
+        --no-anchored                                      \
+        --no-wildcards-match-slash                         \
+        --strip-components=1                               \
+        "*.jar"
+fi