Add devcontainer configuration.

This makes it easy to open the workspace in Ubuntu to reproduce CI
issues locally.

Author: olafurpg

.devcontainer/Dockerfile

@@ -0,0 +1,12 @@
+# See here for image contents: https://github.com/microsoft/vscode-dev-containers/tree/v0.158.0/containers/ubuntu/.devcontainer/base.Dockerfile
+
+# [Choice] Ubuntu version: bionic, focal
+ARG VARIANT="focal"
+FROM mcr.microsoft.com/vscode/devcontainers/base:0-${VARIANT}
+
+# [Optional] Uncomment this section to install additional OS packages.
+RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
+    && apt-get -y install --no-install-recommends default-jdk gradle maven golang-go \
+    && go get github.com/sourcegraph/lsif-semanticdb/cmd/lsif-semanticdb
+
+

.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/v0.158.0/containers/ubuntu
+{
+	"name": "Ubuntu",
+	"build": {
+		"dockerfile": "Dockerfile",
+		// Update 'VARIANT' to pick an Ubuntu version: focal, bionic
+		"args": { "VARIANT": "focal" }
+	},
+
+	// 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": [],
+
+	// 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",
+
+	// Comment out connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
+	"remoteUser": "vscode"
+}