Bump alpine from 3.10.9 to 3.14.3 in /build

Bumps alpine from 3.10.9 to 3.14.3.

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

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

build/Dockerfile

@@ -2,7 +2,7 @@
 
 ARG BINARY=flipt
 
-FROM alpine:3.10.9
+FROM alpine:3.14.3
 LABEL maintainer="mark.aaron.phelps@gmail.com"
 LABEL org.opencontainers.image.name="flipt"
 LABEL org.opencontainers.image.source="https://github.com/markphelps/flipt"