gofumpt provides a supserset of gofmt / go fmt, but not every developer may have
it installed, so for situations where it's not available, fall back to gofmt.
As our code has been formatted with gofumpt already, in most cases contributions
will follow those formatting rules, but in some cases there may be a difference,
which would already be flagged by manual code review, but let's also enable the
gofumpt linter.
With this change, `make fmt` will use gofumpt is available; gofumpt has been
added to the dev-container, so `make -f docker.Makefile fmt` will always use it.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
