fix: integration/Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DEBIAN10-GCC8-347558
- https://snyk.io/vuln/SNYK-DEBIAN10-GCC8-347558
- https://snyk.io/vuln/SNYK-DEBIAN10-GCC8-347558
- https://snyk.io/vuln/SNYK-DEBIAN10-GNUTLS28-2964217
- https://snyk.io/vuln/SNYK-DEBIAN10-ZLIB-2976149

Author: snyk-bot

integration/Dockerfile

@@ -1,6 +1,6 @@
 # This Dockerfile builds a base image for the certificate-transparency-go CloudBuild integration testing.
 # See https://hub.docker.com/_/golang for the set of golang base images.
-FROM golang:buster as ct_testbase
+FROM golang:1-buster as ct_testbase
 
 WORKDIR /testbase