Fix environment variable replacement in image configuration

[Copilot]

• Initial plan - understand environment variable replacement issue
• Fix bug in MergeImageConfig where env vars were appended instead of replaced
• Add comprehensive tests for environment variable replacement behavior
• Parse env var names correctly to enable proper replacement logic
• Use strings.Cut instead of strings.Index for cleaner environment variable parsing

The MergeImageConfig function was incorrectly appending environment variables instead of replacing them when merging image configurations. This caused duplicate environment variables like PATH to appear in the final container image.

Problem

When a user specified environment variables in the image section of a dalec spec:

image:
  env:
    - PATH=/custom/bin:/usr/bin

And the base image already had PATH=/usr/bin:/bin, the result would contain both:

• PATH=/usr/bin:/bin (from base image)
• PATH=/custom/bin:/usr/bin (from spec)

This happened because the environment variable matching logic was comparing the full string ("PATH=/usr/bin:/bin") instead of just the variable name ("PATH").

Solution

Modified the MergeImageConfig function to:

1. Parse environment variable names (the part before =) for proper matching using strings.Cut
2. Replace existing environment variables when the same variable name is found
3. Handle edge cases like environment variables without = signs

Example

Before the fix:

baseEnv := []string{"PATH=/usr/bin:/bin", "HOME=/root"}
specEnv := []string{"PATH=/custom/bin"}
// Result: ["PATH=/usr/bin:/bin", "HOME=/root", "PATH=/custom/bin"] ❌

After the fix:

baseEnv := []string{"PATH=/usr/bin:/bin", "HOME=/root"}
specEnv := []string{"PATH=/custom/bin"}
// Result: ["PATH=/custom/bin", "HOME=/root"] ✅

The fix includes comprehensive tests covering replacement scenarios, appending new variables, and handling edge cases. Updated to use strings.Cut for cleaner, more idiomatic string parsing.

Fixes #456.

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[cpuguy83]

@copilot please use strings.Cut here and at line 122

[Copilot]

Updated to use strings.Cut at both locations for cleaner, more idiomatic string parsing. This replaces the strings.Index approach with a more direct way to split environment variable names from their values. Commit: 1866721