Use Go 1.13 (GA)
As Go 1.13 has been available in App Engine standard environment in Beta since 2019-10 and Go 1.12 will be end-of-support by the end of Feb 2020, we prefer moving from Go 1.12 to 1.13
GA
On 2020-06-11, Go 1.13 went GA:
cf. #21 (closed)
Edited by Takuya Noguchi