Improve CI (#819)

* simplify github actions workflow

no more GOPATH, update Go to 1.15.x

* add script to install golangci-lint

* drop support for Go 1.14

* check docker build in ci

* update alpine linux to 3.12

* update CHANGELOG

* fix golangci-lint installation

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>