Added mono

.gitpod

@@ -1,3 +1,5 @@
+image:
+  file: Dockerfile
 ports:
   - port: 3000
     protocol: "http"

Dockerfile

@@ -0,0 +1,9 @@
+FROM gitpod/workspace-full:latest
+
+USER root
+RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF \
+    && echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list \
+    && apt update \
+    && apt-get update \
+    && apt-get install -y mono-devel \
+    && apt-get clean && rm -rf /var/cache/apt/* && rm -rf /var/lib/apt/lists/* && rm -rf /tmp/*