Olgabot/add ska to container

[olgabot]

Add ska and seqtk dependencies in separate container for https://github.com/nf-core/kmermaid/pull/5

PR checklist

• [x] This comment contains a description of changes (with reason) ~- [ ] If you've fixed a bug or added code that should be tested, add tests!~ ~- [ ] If necessary, also make a PR on the nf-core/kmer-similarity branch on the nf-core/test-datasets repo~ ~- [ ] Ensure the test suite passes (nextflow run . -profile test,docker).~ ~- [ ] Make sure your code lints (nf-core lint .).~
• [ ] Documentation in docs is updated
• [x] CHANGELOG.md is updated
• [ ] README.md is updated

Learn more about contributing: https://github.com/nf-core/kmer-similarity/tree/master/.github/CONTRIBUTING.md