Standardize phrasing to "at runtime" in Compose documentation (#23508)

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## Description

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Standardized the phrasing from **"at run time"** to **"at runtime"** for
consistency with other Docker documentation. No technical content
changes.
## Related issues or tickets

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None.
## Reviews

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- [ ] Technical review
- [x] Editorial review
- [ ] Product review

Signed-off-by: MohammadHasan Akbari <[email protected]>

Author: jarqvi

content/manuals/compose/how-tos/environment-variables/variable-interpolation.md

@@ -12,9 +12,9 @@ aliases:
 
 A Compose file can use variables to offer more flexibility. If you want to quickly switch 
 between image tags to test multiple versions, or want to adjust a volume source to your local
-environment, you don't need to edit the Compose file each time, you can just set variables that insert values into your Compose file at run time.
+environment, you don't need to edit the Compose file each time, you can just set variables that insert values into your Compose file at runtime.
 
-Interpolation can also be used to insert values into your Compose file at run time, which is then used to pass variables into your container's environment
+Interpolation can also be used to insert values into your Compose file at runtime, which is then used to pass variables into your container's environment
 
 Below is a simple example: