Pin versions of Docker images, update comment

[kinow]

Reasons for creating this PR

From the linked discussion, looks like Varnish changed the image and now it cannot be used with its default port 80. This pull request pins the versions of the images, instead of using the latest tag, in Docker Compose.

Also updates the commented text to avoid YAML errors.

Link to relevant issue(s), if any

• https://groups.google.com/g/skosmos-users/c/9mrL0UhPpco
• unprivileged version varnish/docker-varnish#42

Description of the changes in this PR

Known problems or uncertainties in this PR

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if not, explain why below)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
2.6% Duplication

[codecov]

Codecov Report

Merging #1294 (e234938) into master (5d193c2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1294   +/-   ##
=========================================
  Coverage     70.68%   70.68%           
  Complexity     1646     1646           
=========================================
  Files            32       32           
  Lines          3786     3786           
=========================================
  Hits           2676     2676           
  Misses         1110     1110           

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[kinow]

Also added some troubleshooting notes to the Wiki: https://github.com/NatLibFi/Skosmos/wiki/Install-Skosmos-with-Fuseki-in-Docker#troubleshooting

[kinow]

Rebased.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.8% Duplication