Fix comment style in Dockerfile

[b-fein]

Comments in Dockerfiles start with # instead of //.

Resulting error can be seen for example in https://github.com/JabRef/jabref-action/actions/runs/18560395771/job/52907702898

Dockerfile:1
  --------------------
     1 | >>> // Corresponding Dockerfile: https://github.com/JabRef/jabref/blob/main/Dockerfile.jabkit 
     2 |     FROM ghcr.io/jabref/jabkit:edge
     3 |     
  --------------------
  ERROR: failed to build: failed to solve: dockerfile parse error on line 1: unknown instruction: //

[b-fein]

resolved by ce93644 already.