Fix issue with loading background image of WalkXcode theme in built app that was run from Docker

[aleksey925]

Description

Fixed the issue with loading the background image of the WalkXcode theme in the built app that was run from Docker. Without this fix, if you choose a non-default theme, the background image will not load due to an invalid URL.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I've read & comply with the contributing guidelines
• I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• I have made corresponding changes to the documentation (README.md).
• I've checked my modifications for any breaking changes, especially in the config.yml file