Build the binary without cgo (closes #545)

make build now builds with CGO_ENABLED=0, and the image builds through
it. The binary's only cgo code was mattn/go-sqlite3, which
gorm.io/driver/sqlite imports and webhooker never opens: every SQLite
file is opened with modernc.org/sqlite and the connection handed to
gorm. Without cgo, mattn's package compiles to a stub nothing calls.

The Dockerfile builds the binary once. The static relink and its
netgo, osusergo and sqlite_omit_load_extension tags are gone, and with
them the Makefile's GO_LDFLAGS and GO_TAGS, which only the relink used.
The test phase keeps cgo for -race. No README or Dockerfile sentence
says the binary needs cgo any more.

Model: opus-5-5

Makefile

@@ -16,17 +16,6 @@ VERSION ?= $(shell script/version)
 # exists to correct.
 override VERSION := $(or $(strip $(VERSION)),$(shell script/version))
 
-# Extra linker flags and build tags for the build target. The static
-# relink in the Dockerfile passes -extldflags and its build tags through
-# these rather than a -ldflags of its own, so composing flags cannot drop
-# the version stamp.
-#
-# The build target itself always passes -trimpath and -s -w, as the Go
-# Dockerfile in REPO_POLICIES.md does: no build paths, symbol table or
-# debug information in the binary.
-GO_LDFLAGS ?=
-GO_TAGS ?=
-
 bootstrap:
 	@script/bootstrap
 
@@ -54,8 +43,12 @@ fmt-check:
 check:
 	@script/check
 
+# Built with cgo off, -trimpath and -s -w, as the Go Dockerfile in
+# REPO_POLICIES.md does: the binary carries no build paths, symbol table or
+# debug information, and on Linux, the image's included, it is statically
+# linked.
 build: assets
-	go build -trimpath -tags '$(GO_TAGS)' -ldflags '$(strip -s -w -X main.version=$(VERSION) $(GO_LDFLAGS))' -o bin/webhooker ./cmd/webhooker
+	CGO_ENABLED=0 go build -trimpath -ldflags '-s -w -X main.version=$(VERSION)' -o bin/webhooker ./cmd/webhooker
 
 run: build
 	./bin/webhooker