[log] Add debug logging to dockerutil/env.go (#1522)

## Changes

Adds a debug logger to `internal/dockerutil/env.go` to trace Docker `-e`
flag environment variable expansion.

### What was added

- **Logger declaration**: `var logDockerutil =
logger.New("dockerutil:env")` following the project's `pkg:filename`
naming convention
- **4 debug logging calls** in `ExpandEnvArgs`:
  1. Function entry log with input arg count
  2. Per-env-var log when a variable is successfully found and expanded
3. Per-env-var log when a referenced variable is **not found** in the
process environment (key debugging signal)
  4. Completion log with output arg count

### Why this is useful

`ExpandEnvArgs` is called when building Docker command arguments,
converting passthrough `-e VAR_NAME` flags to `-e VAR_NAME=value`.
Without logging, there's no visibility into:
- Which env vars are being passed through to Docker containers
- Which env vars are missing from the gateway process environment (a
common misconfiguration)
- Whether the expansion produced the expected number of args

The "not found" log is especially valuable - it highlights env vars that
are referenced in config but not set in the environment, which is a
common source of unexpected Docker container behavior.

### Debug usage

```bash
# Enable dockerutil debug logging
DEBUG=dockerutil:* ./awmg --config config.toml

# Example output
[dockerutil:env] Expanding env args: input_count=8
[dockerutil:env] Expanding env var: name=GITHUB_PERSONAL_ACCESS_TOKEN
[dockerutil:env] Env var not found in process environment: name=MISSING_VAR
[dockerutil:env] Env args expansion complete: output_count=9
```

### Quality checklist

- [x] Exactly 1 file modified
- [x] No test files modified
- [x] Logger declaration added following `pkg:filename` convention
(`dockerutil:env`)
- [x] Logger arguments don't compute anything or cause side effects
(only passes existing variable names/counts)
- [x] Logging messages are meaningful and helpful
- [x] No duplicate logging with existing logs
- [x] Import statements properly formatted with stdlib imports before
external imports




> Generated by [Go Logger
Enhancement](https://github.com/github/gh-aw-mcpg/actions/runs/22523661602)

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Author: lpcox

internal/dockerutil/env.go

@@ -4,11 +4,16 @@ import (
 	"fmt"
 	"os"
 	"strings"
+
+	"github.com/github/gh-aw-mcpg/internal/logger"
 )
 
+var logDockerutil = logger.New("dockerutil:env")
+
 // ExpandEnvArgs expands Docker -e flags that reference environment variables
 // Converts "-e VAR_NAME" to "-e VAR_NAME=value" by reading from the process environment
 func ExpandEnvArgs(args []string) []string {
+	logDockerutil.Printf("Expanding env args: input_count=%d", len(args))
 	result := make([]string, 0, len(args))
 	for i := 0; i < len(args); i++ {
 		arg := args[i]
@@ -20,14 +25,17 @@ func ExpandEnvArgs(args []string) []string {
 			if len(nextArg) > 0 && !strings.Contains(nextArg, "=") {
 				// Look up the variable in the environment
 				if value, exists := os.LookupEnv(nextArg); exists {
+					logDockerutil.Printf("Expanding env var: name=%s", nextArg)
 					result = append(result, "-e")
 					result = append(result, fmt.Sprintf("%s=%s", nextArg, value))
 					i++ // Skip the next arg since we processed it
 					continue
 				}
+				logDockerutil.Printf("Env var not found in process environment: name=%s", nextArg)
 			}
 		}
 		result = append(result, arg)
 	}
+	logDockerutil.Printf("Env args expansion complete: output_count=%d", len(result))
 	return result
 }