Add docker image publish GH workflow

[YuviGold]

Following

• https://github.com/asciinema/agg/issues/53

Using the same workflow as in asciinema-server

• https://github.com/asciinema/asciinema-server/commit/36755d558ef473b6a1b42b5265c72bc6ad1ca04c

[ku1ik]

Cool, thanks! Let's try this :+1: