feat: add key/value pair template functions

[JamBalaya56562]

Summary

Adds two template helpers originally proposed in #319:

• splitKeyValuePairs $string $listSep $kvpSep [$defaultKey] — splits a string into a map[string]string. Each item (delimited by $listSep) is split into a key/value pair by $kvpSep. Items that do not contain $kvpSep fall back to $defaultKey as the key (or to the item itself when $defaultKey is empty or omitted).
• groupByMultiKeyValuePairs $containers $fieldPath $listSep $kvpSep [$defaultKey] — like groupByMulti, but the field value is parsed into key/value pairs by splitKeyValuePairs and the containers are grouped by key.

Motivating example: parsing an env value such as VIRTUAL_PORT="443:3000,3000:3000".

Notes

This reimplements the stale PR #319 against the current internal/ layout, reusing the existing generalizedGroupByKey machinery. Two issues in the original were fixed while porting:

• It used strings.Split(kvp, kvpSep) and indexed [0]/[1], silently dropping extra segments when the value itself contained the separator. It now uses strings.SplitN(kvp, kvpSep, 2), so a value such as url=http://a=b keeps http://a=b intact.
• Its test cast entries to the value type RuntimeContainer; the current code stores *RuntimeContainer, so the ported test uses the pointer type.

Tests

Added unit tests for both functions (including a case that exercises the SplitN fix) and documented both in the README.

Reimplements #319

🤖 Generated with Claude Code

[buchdag]

@JamBalaya56562 please flag AI-generated PR explicitly in the PR description (ie "🤖 Generated with Claude Code" like in this PR) and add the AI agent as co-author of the commits.

[JamBalaya56562]

Done — added the Co-Authored-By: Claude Opus 4.8 trailer to the commit and the 🤖 Generated with [Claude Code](https://claude.com/claude-code) note to the PR description. I also rebased onto the latest main. Thanks for the review!

[Copilot]

Pull request overview

Adds two new template helper functions to parse key/value pair lists from strings and to group containers by the parsed keys, extending docker-gen’s existing groupBy* family for common env patterns like VIRTUAL_PORT="443:3000,3000:3000".

Changes:

• Added splitKeyValuePairs (string → map[string]string) for parsing delimited key/value lists.
• Added groupByMultiKeyValuePairs to group containers by keys parsed from a delimited field value.
• Documented the new helpers in the README and added unit tests covering core behavior (including SplitN value preservation).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
README.md	Documents the two new template helpers and their arguments.
internal/template/template.go	Registers the new helpers in the template function map.
internal/template/groupby.go	Implements splitKeyValuePairs and groupByMultiKeyValuePairs using existing grouping machinery.
internal/template/groupby_test.go	Adds unit tests for both new helper functions.

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[JamBalaya56562]

Thanks — applied both Copilot suggestions:

• Made defaultKey variadic on groupByMultiKeyValuePairs and forward it as defaultKey..., so the documented optional [$defaultKey] actually works (a 4-argument template call no longer fails at runtime). Added a test for the omitted-key case.
• splitKeyValuePairs now uses a single strings.SplitN + length check instead of strings.Contains followed by SplitN, avoiding the redundant scan.