Bump alpine from 3.12 to 3.13.1 (#117)

Bumps alpine from 3.12 to 3.13.1.

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

Author: dependabot[bot]

Dockerfile

@@ -8,6 +8,6 @@ COPY internal/ internal
 ARG KONSTRAINT_VER
 RUN go build -o /konstraint -ldflags="-X 'github.com/plexsystems/konstraint/internal/commands.version=${KONSTRAINT_VER}'"
 
-FROM alpine:3.12
+FROM alpine:3.13.1
 COPY --from=build /konstraint /usr/bin/konstraint
 ENTRYPOINT ["konstraint"]