Provide official docker image `dioxus/dx`

I'd like to work on this.

## Specific Demand

It's common to pack applications into container images in CI/CD and DevOps, and I do pack a dioxus application into a docker image. It could be helpful if a docker image is provided officially and usable as a base image for a build stage in a Dockerfile. It took me a half-hour to get my Dockerfile working, and I wish I had an official image for the `dx` command.

## Implement Suggestion

Something like:

```dockerfile
FROM rust
RUN apt-get update && apt-get install -y pkg-config libssl-dev perl make \
&& cargo install dioxus-cli --locked && rustup target add wasm32-unknown-unknown
```

cf. https://github.com/rust-lang/docker-rust