Bump elixir from 1.15.4-alpine to 1.15.6-alpine

Bumps elixir from 1.15.4-alpine to 1.15.6-alpine.

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

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

Dockerfile

@@ -1,4 +1,4 @@
-FROM elixir:1.15.4-alpine AS build
+FROM elixir:1.15.6-alpine AS build
 # install build dependencies
 RUN apk add --no-cache build-base npm git python3
 # prepare build dir