This now allows us to use things like the ProjectName as variables
in our docker image name. This is especially helpful when trying to
make the `.goreleaser.yml` as generic as possible.
I enhanced `BuildFlagTemplates` field in Docker image config to accept
templates. Build flags are tested by checking for images created with
labels applied through templates.
See #813
I added `BuildFlags` field to Docker image config. Docker pipe is
updated to append build flags to the `docker build` command.
Build flags are not validated, leaving it Docker to error
when given an unknown flag.
See #813
