travis: default to buster and also use debian:buster as docker container

.travis.yml

@@ -5,7 +5,7 @@ services:
   - docker
 
 env:
-  - TRAVIS_DEBIAN_DISTRIBUTION=stretch
+  - TRAVIS_DEBIAN_DISTRIBUTION=buster
   - TRAVIS_DEBIAN_DISTRIBUTION=unstable TRAVIS_DEBIAN_INCREMENT_VERSION_NUMBER=true
 
 script:

travis/execute.sh

@@ -18,7 +18,7 @@ if [ -z "${TRAVIS:-}" ] ; then
   fi
 fi
 
-RELEASE="${RELEASE:-stretch}"
+RELEASE="${RELEASE:-buster}"
 export RELEASE
 
 TARGET="${TARGET:-qemu.img}"
@@ -69,7 +69,7 @@ else
 fi
 
 # we need to run in privileged mode to be able to use loop devices
-docker run --privileged -v "$(pwd)":/code --rm -i -t debian:stretch /code/travis/build-vm.sh
+docker run --privileged -v "$(pwd)":/code --rm -i -t debian:buster /code/travis/build-vm.sh
 
 [ -x ./goss ] || curl -fsSL https://goss.rocks/install | GOSS_DST="$(pwd)" sh