Update to Go 1.18

It is already required in practice, see
> grep 'func .*]('

This also renders the -compat=1.17 option to (go mod tidy)
unnecessary, so drop it.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

.cirrus.yml

@@ -121,7 +121,7 @@ vendor_task:
 
     # Runs within Cirrus's "community cluster"
     container:
-        image: docker.io/library/golang:1.17
+        image: docker.io/library/golang:1.18
         cpu: 1
         memory: 1
 

Makefile

@@ -185,11 +185,11 @@ test-unit: tests/testreport/testreport
 	$(GO_TEST) -v -tags "$(STORAGETAGS) $(SECURITYTAGS)" -cover $(RACEFLAGS) ./cmd/buildah -args --root $$tmp/root --runroot $$tmp/runroot --storage-driver vfs --signature-policy $(shell pwd)/tests/policy.json --registries-conf $(shell pwd)/tests/registries.conf
 
 vendor-in-container:
-	podman run --privileged --rm --env HOME=/root -v `pwd`:/src -w /src docker.io/library/golang:1.17 make vendor
+	podman run --privileged --rm --env HOME=/root -v `pwd`:/src -w /src docker.io/library/golang:1.18 make vendor
 
 .PHONY: vendor
 vendor:
-	GO111MODULE=on $(GO) mod tidy -compat=1.17
+	GO111MODULE=on $(GO) mod tidy
 	GO111MODULE=on $(GO) mod vendor
 	GO111MODULE=on $(GO) mod verify
 

go.mod

@@ -1,6 +1,6 @@
 module github.com/containers/buildah
 
-go 1.17
+go 1.18
 
 require (
 	github.com/containerd/containerd v1.7.0