Bump alpine from 3.14.3 to 3.15.0 in /.ci/docker-ci/alpine (#737)

Bumps alpine from 3.14.3 to 3.15.0.

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.ci/docker-ci/alpine/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.14.3
+FROM alpine:3.15.0
 
 LABEL maintainer="mail@sobolevn.me"
 LABEL vendor="git-secret team"