Add docs and configs for VS Code (#60)

* Add .devcontainer to .gitignore

* Add VS Code docs and configs

* Add extensions.json

* Refine docs

* Update README.md

.devcontainer.example/Dockerfile

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+FROM mcr.microsoft.com/dotnet/sdk:5.0
+
+# [Option] Install zsh
+ARG INSTALL_ZSH="true"
+# [Option] Upgrade OS packages to their latest versions
+ARG UPGRADE_PACKAGES="false"
+# [Option] Enable non-root Docker access in container
+ARG ENABLE_NONROOT_DOCKER="true"
+# [Option] Use the OSS Moby Engine instead of the licensed Docker Engine
+ARG USE_MOBY="true"
+
+# Install needed packages and setup non-root user. Use a separate RUN statement to add your
+# own dependencies. A user of "automatic" attempts to reuse an user ID if one already exists.
+ARG USERNAME=automatic
+ARG USER_UID=1000
+ARG USER_GID=$USER_UID
+COPY library-scripts/*.sh /tmp/library-scripts/
+RUN apt-get update \
+    && /bin/bash /tmp/library-scripts/common-debian.sh "${INSTALL_ZSH}" "${USERNAME}" "${USER_UID}" "${USER_GID}" "${UPGRADE_PACKAGES}" \
+    # Use Docker script from script library to set things up
+    && /bin/bash /tmp/library-scripts/docker-in-docker-debian.sh "${ENABLE_NONROOT_DOCKER}" "${USERNAME}" "${USE_MOBY}" \
+    # Clean up
+    && apt-get autoremove -y && apt-get clean -y && rm -rf /var/lib/apt/lists/* /tmp/library-scripts/
+
+VOLUME [ "/var/lib/docker" ]
+
+# Setting the ENTRYPOINT to docker-init.sh will start up the Docker Engine 
+# inside the container "overrideCommand": false is set in devcontainer.json. 
+# The script will also execute CMD if you need to alter startup behaviors.
+ENTRYPOINT [ "/usr/local/share/docker-init.sh" ]
+CMD [ "sleep", "infinity" ]
+
+# Instructions to install latest Mono from
+# https://www.mono-project.com/download/stable/#download-lin-debian
+RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF && \
+    echo "deb https://download.mono-project.com/repo/debian stable-buster main" | tee /etc/apt/sources.list.d/mono-official-stable.list && \
+    apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
+        mono-complete
+
+# Instructions to install .NET Core runtimes from
+# https://docs.microsoft.com/en-us/dotnet/core/install/linux-package-manager-debian10
+RUN wget https://packages.microsoft.com/config/debian/10/packages-microsoft-prod.deb -O packages-microsoft-prod.deb && \
+    dpkg -i packages-microsoft-prod.deb && \
+    apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
+        apt-transport-https \
+        aspnetcore-runtime-2.1 \
+        aspnetcore-runtime-3.0 \
+        aspnetcore-runtime-3.1
+
+# [Optional] Uncomment this section to install additional OS packages.
+# RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
+#     && apt-get -y install --no-install-recommends <your-package-list-here>

.devcontainer.example/devcontainer.json

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+// For format details, see https://aka.ms/devcontainer.json. For config options, see the README at:
+// https://github.com/microsoft/vscode-dev-containers/tree/v0.158.0/containers/docker-in-docker
+{
+	"name": "Docker in Docker",
+	"dockerFile": "Dockerfile",
+	"runArgs": ["--init", "--privileged"],
+	"overrideCommand": false,
+
+	// Use this environment variable if you need to bind mount your local source code into a new container.
+	// Set *default* container specific settings.json values on container create.
+	"settings": { 
+		"terminal.integrated.shell.linux": "/bin/zsh"
+	},
+
+	// Add the IDs of extensions you want installed when the container is created.
+	"extensions": [
+		"ms-azuretools.vscode-docker",
+		"ms-dotnettools.csharp"
+	],
+
+	// Use 'forwardPorts' to make a list of ports inside the container available locally.
+	// "forwardPorts": [],
+
+	// Use 'postCreateCommand' to run commands after the container is created.
+	"postCreateCommand": "dotnet build Datadog.Trace.Minimal.sln || true",
+
+	// Comment out connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
+	// "remoteUser": "vscode"
+}