Trying to fix Unity versions availability issue

DragonBox/u3d#334
DragonBox/u3d#360

Author: KonH

Docker/Dockerfile

@@ -77,7 +77,16 @@ RUN apt-get install gnupg ca-certificates \
     && rm -rf /var/lib/apt/lists/*
 
 # U3D for further unity install
-RUN gem install u3d -v '1.1.5' && gem list
+# Oficial way
+# RUN gem install u3d -v '1.1.5' && gem list
+# Trying to fix available issue
+RUN git clone https://github.com/niezbop/u3d.git \
+    && cd u3d \
+    && git checkout -b custom_download_page origin/feature/custom_download_page \
+    && git log -1 \
+    && chmod a+x build.sh \
+    && ./build.sh
+RUN u3d -v
 
 # Butler for itch.io upload
 RUN wget -O butler.zip https://broth.itch.ovh/butler/linux-amd64/LATEST/archive/default \