Bump alpine from 3.15.0 to 3.15.1 in /build (flipt-io#757)

Bumps alpine from 3.15.0 to 3.15.1.

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

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

build/Dockerfile

@@ -2,7 +2,7 @@
 
 ARG BINARY=flipt
 
-FROM alpine:3.15.0
+FROM alpine:3.15.1
 LABEL maintainer="mark.aaron.phelps@gmail.com"
 LABEL org.opencontainers.image.name="flipt"
 LABEL org.opencontainers.image.source="https://github.com/markphelps/flipt"