waitfordb: initial tool — env-configured wait-for-DB init container

Small Go tool + distroless container used as a K8s initContainer to block an
app until its database (Postgres/MySQL) is reachable. Env-var configured
(WAITFORDB_* + libpq PG* fallback), configurable timeout/interval, redacted
logs, exit codes. Woodpecker CI publishes docker-internal/waitfordb on tag.

internal/driver/sql.go

@@ -0,0 +1,31 @@
+package driver
+
+import (
+	"context"
+	"database/sql"
+)
+
+// sqlPinger runs the liveness query against a database/sql handle. It is shared
+// by every engine whose Go driver plugs into database/sql.
+type sqlPinger struct {
+	db *sql.DB
+}
+
+func openSQL(driverName, dsn string) (*sqlPinger, error) {
+	db, err := sql.Open(driverName, dsn)
+	if err != nil {
+		return nil, err
+	}
+	// A readiness check only ever needs one connection; keep the pool tiny so a
+	// failed attempt does not leave idle half-open connections behind.
+	db.SetMaxOpenConns(1)
+	db.SetMaxIdleConns(0)
+	return &sqlPinger{db: db}, nil
+}
+
+func (p *sqlPinger) Ping(ctx context.Context) error {
+	var one int
+	return p.db.QueryRowContext(ctx, "SELECT 1").Scan(&one)
+}
+
+func (p *sqlPinger) Close() error { return p.db.Close() }