ci: build container on release (#396)

* new (CI): optimized Dockerfile; labeled Dockerfile; Github Action to build container; edit docker-compose

* new (CI): rename secrets

* CI: edit registry name

* CI: edit registry namespace

* CI: do repeat yourself in GHA

* CI: fix maintainers; debug version tag

* CI: add rpc to env file

* CI: build the same image in parallel, what a joke

* CI: fix matrix

* CI: debug matrix

* CI: debug matrix 2

* CI: according to doc this should work

* CI: proudly move it to parity namespace

* CI: add docker pull to README