perf: Faster mono installation (#8)

avoid running apt update twice
cleanup packages after installing mono

Dockerfile

@@ -9,8 +9,11 @@ LABEL "repository"="http://github.com/MirrorNG/unity-runner"
 LABEL "homepage"="http://github.com/MirrorNG/unity-runner"
 LABEL "maintainer"="Paul Pacheco <paulpach@gmail.com>"
 
-RUN apt-get update && \
-    apt-get install -y --no-install-recommends default-jre unzip && \
+RUN apt install gnupg ca-certificates && \
+    apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF && \
+    echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | tee /etc/apt/sources.list.d/mono-official-stable.list && \
+    apt-get update && \
+    apt-get install -y --no-install-recommends default-jre unzip mono-devel && \
     rm -rf /var/lib/apt/lists/* /var/cache/apt/archives/* && \
     apt-get autoremove -y 
 
@@ -20,19 +23,11 @@ RUN /opt/Unity/Editor/Data/NetCore/Sdk-2.2.107/dotnet tool install dotnet-sonars
 COPY unity_csc.sh.patch .
 RUN patch /opt/Unity/Editor/Data/Tools/RoslynScripts/unity_csc.sh unity_csc.sh.patch
 
-# install mono
-RUN apt install gnupg ca-certificates && \
-    apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF && \
-    echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | tee /etc/apt/sources.list.d/mono-official-stable.list && \
-    apt update && \
-    apt install -y mono-devel
-
 # install docfx
 RUN wget https://github.com/dotnet/docfx/releases/download/v2.56.6/docfx.zip && \
     unzip docfx.zip -d /docfx && \
     rm docfx.zip
 
-
 COPY entrypoint.sh activate.sh sonar-scanner.sh request_activation.sh docfx.sh /
 
 ENV DOTNET_ROOT=/opt/Unity/Editor/Data/NetCore/Sdk-2.2.107/