Add devcontainer (#1040)

* Add devcontainer (generic add-on devcontainer also available at https://github.com/issacg/hassio-addon-devcontainer)

* remove example code (https://github.com/home-assistant/hassio-addons/pull/1040#discussion_r374373037)

* Run start_hassio.sh from inside .devcontainer (https://github.com/home-assistant/hassio-addons/pull/1040#discussion_r374373323)

.devcontainer/Dockerfile

@@ -0,0 +1,34 @@
+FROM ubuntu:18.04
+
+WORKDIR /workspaces
+
+# Default ENV
+ENV LANG C.UTF-8
+ENV DEBIAN_FRONTEND noninteractive
+
+# Set shell
+SHELL ["/bin/bash", "-o", "pipefail", "-c"]
+
+# Install docker
+# https://docs.docker.com/engine/installation/linux/docker-ce/ubuntu/
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        apt-transport-https \
+        ca-certificates \
+        curl \
+        dbus \
+        software-properties-common \
+        gpg-agent \
+        git \
+        sudo \
+    && curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add - \
+    && add-apt-repository "deb https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" \
+    && apt-get update && apt-get install -y --no-install-recommends \
+        docker-ce \
+        docker-ce-cli \
+        containerd.io
+# This is a development container.  Don't bother to clean up apt cache, this way we have it handy later
+
+# Generate a machine-id for this container
+RUN rm /etc/machine-id && dbus-uuidgen --ensure=/etc/machine-id
+
+ENV DEBIAN_FRONTEND=dialog