1. Explain how to build `bb` for linux using docker.
2. Add support for setting the various `BABASHKA_FEATURE_*`
when doing docker build via `--build-arg`.
With the current Ubuntu based docker image it is not possible to use
babashka.curl:
```
> docker run --rm -it borkdude/babashka:0.2.3-SNAPSHOT bb '(-> "https://httpstat.us/200" babashka.curl/get :status)'
----- Error --------------------------------------------------------------------
Type: java.io.IOException
Message: Cannot run program "curl": error=2, No such file or directory
Location: <expr>:1:1
----- Context ------------------------------------------------------------------
1: (-> "https://httpstat.us/200" babashka.curl/get :status)
^--- Cannot run program "curl": error=2, No such file or directory
----- Stack trace --------------------------------------------------------------
babashka.curl/get - <built-in>
user - <expr>:1:1
```
After this change it works:
```
> docker run --rm -it hansbugge/babashka bb '(->
"https://httpstat.us/200" babashka.curl/get :status)'
200
```
