Create new image `releng-scripts`

[chevdor]

A new image is required for https://github.com/paritytech/releng-scripts/blob/master/Dockerfile Since the image will have access to secrets such as publishing to S3, the security needs to be checked carefully.

• [x] make it a rootless image
• [x] harden image
• [x] add Dockerfile to this repo

[chevdor]

See work done in https://github.com/paritytech/releng-scripts/pull/34

[chevdor]

Done, see in https://github.com/paritytech/ci_cd/issues/737