As it is difficult to find special characters with the current search machines,
it took a while to finding the solution:
A ResourcePatternResolver implementation that is able to resolve a
specified resource location path into one or more matching Resources.
The source path may be a simple path which has a one-to-one mapping to
a target Resource, or alternatively may contain the special
"classpath*:" prefix and/or internal Ant-style regular expressions
(matched using Spring's AntPathMatcher utility). Both of the latter
are effectively wildcards.
taken from: https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/core/io/support/PathMatchingResourcePatternResolver.html
So, the solution is (mind the asterisk):
spring.datasource.data=classpath:/data.sql,classpath*:/init-*.sql