fix: add server_url attribute to github context

[KnisterPeter]

The server_urL attribute was missing in the github context.
Previously it was exposed as environment variable only.

Closes #1726

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ EDITORCONFIG	editorconfig-checker	2		0	0.02s
✅ REPOSITORY	gitleaks	yes		no	2.51s
✅ REPOSITORY	git_diff	yes		no	0.0s
✅ REPOSITORY	secretlint	yes		no	1.2s

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[codecov]

Codecov Report

Merging #1727 (6e1f5eb) into master (4989f44) will increase coverage by 1.23%.
The diff coverage is 69.44%.

@@            Coverage Diff             @@
##           master    #1727      +/-   ##
==========================================
+ Coverage   61.22%   62.46%   +1.23%     
==========================================
  Files          46       48       +2     
  Lines        7141     7523     +382     
==========================================
+ Hits         4372     4699     +327     
- Misses       2462     2502      +40     
- Partials      307      322      +15     

Impacted Files	Coverage Δ
pkg/common/outbound_ip.go	0.00% <0.00%> (ø)
pkg/container/docker_cli.go	82.23% <ø> (ø)
pkg/container/docker_logger.go	52.08% <ø> (ø)
pkg/container/docker_pull.go	33.33% <ø> (ø)
pkg/container/docker_run.go	14.08% <0.00%> (+0.49%)	⬆️
pkg/container/docker_volume.go	0.00% <ø> (ø)
pkg/container/file_collector.go	39.68% <0.00%> (+2.38%)	⬆️
pkg/container/host_environment.go	0.00% <0.00%> (ø)
pkg/exprparser/functions.go	66.32% <0.00%> (-1.04%)	⬇️
pkg/model/workflow.go	42.11% <ø> (ø)
... and 21 more

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