add docker devcontainer

.devcontainer/Dockerfile

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+FROM node:8
+RUN npm install -g typescript@2.9.2
+RUN npm install -g firebase-tools@8.10.0

.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/microsoft/vscode-dev-containers/tree/v0.140.1/containers/typescript-node
+{
+	"name": "Peppermint Container",
+	"build": {
+		"dockerfile": "Dockerfile"
+	},
+
+	// Set *default* container specific settings.json values on container create.
+	"settings": {
+		"terminal.integrated.shell.linux": "/bin/bash"
+	},
+
+	// Add the IDs of extensions you want installed when the container is created.
+	"extensions": [
+		"dbaeumer.vscode-eslint",
+		"ms-vscode.vscode-typescript-tslint-plugin"
+	],
+
+	// 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": "npm i",
+
+	// Uncomment to connect as a non-root user. See https://aka.ms/vscode-remote/containers/non-root.
+	// "remoteUser": "node"
+}
+