Resolves: Add GitHub Codespaces configuration (restsharp#1599)

* Create Codespace configuration (#14)

* reduce Docker weight (#15)

Author: aleks-ivanov

.devcontainer/Dockerfile

@@ -0,0 +1,18 @@
+FROM mcr.microsoft.com/vscode/devcontainers/dotnet:0.201.7-5.0
+
+# Install Mono for running tests
+RUN sudo apt install gnupg ca-certificates && \
+	sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF && \
+	echo "deb https://download.mono-project.com/repo/ubuntu stable-focal main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list && \
+	sudo apt update && \
+	sudo apt install -y mono-complete && \
+# Install .NET Core 3.1 for running tests
+  sudo apt-get install wget && \
+	wget https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb && \
+	sudo dpkg -i packages-microsoft-prod.deb && \
+	sudo apt-get update && \
+	sudo apt-get install -y apt-transport-https && \
+	sudo apt-get update && \
+	sudo apt-get install -y dotnet-sdk-3.1
+
+# Built with ❤ by [Pipeline Foundation](https://pipeline.foundation)

.devcontainer/devcontainer.json

@@ -0,0 +1,27 @@
+{
+	"name": "RestSharp Codespace",
+	"settings": {
+		"workbench.colorTheme": "Default Dark+",
+		"terminal.integrated.defaultProfile.linux": "pwsh"
+	},
+	"extensions": [
+		"eamodio.gitlens",
+		"ms-dotnettools.csharp",
+		"VisualStudioExptTeam.vscodeintellicode",
+		"ms-vscode.powershell",
+		"cschleiden.vscode-github-actions",
+		"redhat.vscode-yaml",
+		"bierner.markdown-preview-github-styles",
+		"ban.spellright",
+		"jmrog.vscode-nuget-package-manager",
+		"coenraads.bracket-pair-colorizer",
+		"vscode-icons-team.vscode-icons",
+		"editorconfig.editorconfig"
+	],
+	"postCreateCommand": "dotnet restore RestSharp.sln && dotnet build RestSharp.sln --configuration Release --no-restore && dotnet test RestSharp.sln --configuration Release --no-build",
+	"build": {
+		"dockerfile": "Dockerfile"
+	}
+}
+
+// Built with ❤ by [Pipeline Foundation](https://pipeline.foundation)

.gitignore

@@ -52,3 +52,4 @@ RestSharp.IntegrationTests/config.json
 /node_modules/
 /out/
 /docs/.vuepress/dist/
+.vscode/