build(deps): bump composer from 2.0 to 2.7 in /build (#40)

Bumps composer from 2.0 to 2.7.

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

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

build/Dockerfile

@@ -2,7 +2,7 @@
 ARG PHP=7.3
 
 # prepare PHP source with dependencies
-FROM composer:2.0 as vendor
+FROM composer:2.7 as vendor
 
 ARG JEEDOM_VERSION=V4-stable
 ENV JEEDOM_VERSION=${JEEDOM_VERSION}