`Dockerfile` improvements

[yafetn]

Hello,

Thanks for putting out these samples and for the documentation.

I wanted to make some changes to the Dockerfiles but thought to open an issue first. The proposed changes are:

1. To not run the containers as root but as a separate user (maybe aeron)
2. Decrease the repetitiveness of COPY... statements by using .dockerignore files
3. Replace wget .... in favor of ADD... commands.

Let me know if these sound reasonable and I'll be happy to open a PR.

[eleventy7]

I've made some initial changes around the first point - the applications in the containers no longer run under root but now aeron, as suggested.

[yafetn]

Awesome, thank you!