Update dockerfile for latest mono

Dockerfile

@@ -1,11 +1,11 @@
-FROM mono:5.4.0.201 as builder
+FROM mono:5.12 as builder
 COPY . /tmp/
 WORKDIR /tmp
-RUN msbuild /t:restore /t:build /p:Configuration=Release && \
-    rm /tmp/OhmGraphite/bin/Release/net46/OhmGraphite.exe.config
+RUN msbuild /t:restore /t:build /p:Configuration=Release;TargetFrameworkVersion=v4.6.1 && \
+    rm /tmp/OhmGraphite/bin/Release/net461/OhmGraphite.exe.config
 
-FROM mono:5.4.0.201
-COPY --from=builder /tmp/OhmGraphite/bin/Release/net46 /opt/OhmGraphite
+FROM mono:5.12
+COPY --from=builder /tmp/OhmGraphite/bin/Release/net461 /opt/OhmGraphite
 COPY --from=builder /tmp/OhmGraphite/NLog.docker.config /opt/OhmGraphite/NLog.config
 WORKDIR /opt/OhmGraphite
 VOLUME /opt/OhmGraphite/OhmGraphite.exe.config