Docker development image with Azure CLI + functions tools

Create a Docker development image with the Azure CLI and functions tools already installed.

Here's a non-working starter draft:

```
FROM mcr.microsoft.com/azure-cli

ARG TOOLS_URL="https://github.com/Azure/azure-functions-core-tools/releases/download/3.0.2798/Azure.Functions.Cli.linux-x64.3.0.2798.zip"

RUN apk update \
&& apk add --no-cache wget

WORKDIR /tmp

RUN curl -L -o az-func.zip $TOOLS_URL \
&& unzip -d /usr/local/bin/func az-func.zip \
&& rm -rf az-func.zip

RUN func --version
```