docs: replaces openjdk-alpine with slim image on SAST docs

What does this MR do?

Replaces openjdk-alpine maven:3.6-jdk-8-alpine with slim image maven:3.6-jdk-8-slim on SAST docs.

As a minor change, globally defined image was moved to job-specific one since globally defined image has been inherited in only build job. cf. deprecation of globally defined image

Why

https://github.com/carlossg/docker-maven/commit/6251fd3dfc1a3ec50c7c38a112acc5d6b3579dc5 deprecated alpine-based openjdk followed by the upstream policy change at https://github.com/docker-library/openjdk/pull/322. So with maven:3.6-jdk-8-alpine tag, we have not got any update on OpenJDK for 2 years. We can use maven:3.6-jdk-8-slim tag to get the JDK updates.

Considerations

slim image could be larger than alpine image:

$ docker pull maven:3.6-jdk-8-alpine
...
$ docker pull maven:3.6-jdk-8-slim
...
$ docker image list maven
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
maven               3.6-jdk-8-slim      2b7040144035        3 weeks ago         306MB
maven               3.6-jdk-8-alpine    7445f83cd169        22 months ago       122MB

Actually 2.5x increased in the size, but there is no other solution than this, so this could be acceptable since this is a single job among the pipeline.

Related issues

Closes #323898 (closed)

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