ci: Update Docker image to Go 1.13
ci: Update Docker image to Go 1.13

- The Go version now matches the one used for the GHA CI jobs
- The Docker builds failed because a dependency changed and Go 1.12 was not able to use the version from go.sum (see https://hub.docker.com/repository/registry-1.docker.io/tusproject/tusd/builds/913de821-d650-454b-9f05-107862caedf3)
tus/tusd,tus/tusd