Add Dockerfile and devcontainer.json for universal template

templates/universal/.devcontainer/Dockerfile

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+FROM mcr.microsoft.com/devcontainers/universal:2-linux
+
+# config fish as default shell
+RUN chsh -s /usr/bin/fish
+
+# install oh-my-fish
+RUN curl -L https://get.oh-my.fish | fish
+
+RUN omf theme agnoster
+

templates/universal/.devcontainer/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/devcontainers/templates/tree/main/src/universal
+{
+	"name": "Default Linux Universal",
+	// Or use a Dockerfile or Docker Compose file. More info: https://containers.dev/guide/dockerfile
+	"image": "mcr.microsoft.com/devcontainers/universal:2-linux",
+	"customizations": {
+		"vscode": {
+			"extensions": [
+				"npv2k1.theme-dracula-gray",
+				"ms-azuretools.vscode-docker"
+			]
+		}
+	}
+
+	// Features to add to the dev container. More info: https://containers.dev/features.
+	// "features": {},
+
+	// 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": "uname -a",
+
+	// Configure tool-specific properties.
+	// "customizations": {},
+
+	// Uncomment to connect as root instead. More info: https://aka.ms/dev-containers-non-root.
+	// "remoteUser": "root"
+}

templates/universal/.github/dependabot.yml

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+# To get started with Dependabot version updates, you'll need to specify which
+# package ecosystems to update and where the package manifests are located.
+# Please see the documentation for more information:
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+# https://containers.dev/guide/dependabot
+
+version: 2
+updates:
+ - package-ecosystem: "devcontainers"
+   directory: "/"
+   schedule:
+     interval: weekly