Upgrade alpine image to 3.12

.travis.yml

@@ -9,7 +9,7 @@ script:
 - docker build stretch/slim
 - docker build buster
 - docker build buster/slim
-- docker build alpine3.10
+- docker build alpine3.12
 after_success: |
   if [ "$TRAVIS_BRANCH" = "master" -a "$TRAVIS_PULL_REQUEST" = "false" ]; then
     curl -sS -H "Content-Type: application/json" --data '{"build": true}' -X POST \

alpine3.10/Dockerfile → alpine3.12/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.10
+FROM alpine:3.12
 
 ENV RUSTUP_HOME=/usr/local/rustup \
     CARGO_HOME=/usr/local/cargo \