ci: add Gitea Actions workflow for make check (#21)

Adds CI workflow that runs `make check` on push/PR to main.

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: clawbot <clawbot@eeqj.de>
Reviewed-on: #21
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>
Co-committed-by: clawbot <sneak+clawbot@sneak.cloud>

Makefile

@@ -17,7 +17,7 @@ build: ./secret
 vet:
 	go vet ./...
 
-test: lint vet
+test: vet
 	go test ./... || go test -v ./...
 
 fmt:
@@ -26,7 +26,7 @@ fmt:
 lint:
 	golangci-lint run --timeout 5m
 
-check: build test
+check: build lint test fmt-check
 
 # Build Docker container
 docker:
@@ -42,3 +42,6 @@ clean:
 
 install: ./secret
 	cp ./secret $(HOME)/bin/secret
+
+fmt-check:
+	@test -z "$$(gofmt -l .)" || (echo "Files need formatting:" && gofmt -l . && exit 1)