Bump alpine from 3.14.3 to 3.15.0 in /build (#599)

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>

build/Dockerfile

@@ -2,7 +2,7 @@
 
 ARG BINARY=flipt
 
-FROM alpine:3.14.3
+FROM alpine:3.15.0
 LABEL maintainer="mark.aaron.phelps@gmail.com"
 LABEL org.opencontainers.image.name="flipt"
 LABEL org.opencontainers.image.source="https://github.com/markphelps/flipt"