dockerfile-and-readme-templating

# Instructions for modifying Dockerfile templates All Dockerfiles and Readmes are generated from [Cottle](https://github.com/r3c/cottle) templates in the `eng/dockerfile-templates` and `eng/readme-templates` directories respectively. ## Cottle syntax ### Templating All text is plain text by default. Text inside double curly braces `{{ }}` is template code. Writing just `{{value}}` emits the evaluated value. You can also have expressions inside template blocks which do more complicated logic. Template code can have multiple lines in it, separated by the line ending `^` character. The final line in a template block should not use the line ending character. Comments inside a template block start with `_` and follow the same line ending rules. Comments can span multiple lines. ### Template data sources - `VARIABLES`: Variables from `manifest.versions.json`. Access with `VARIABLES["name"]`. - `ARGS`: Passed into template via `InsertTemplate(...)` function. Access with `ARGS["name"]` ### Common functions - Assign value: `set name to <expression>`. - Assign function `set func(arg1, arg2) to:{{ ... }}` defines an inline function; use `return <expr>` inside. - String functions: - `cat(a, b, ...)` - `join(list, sep)` - `split(str, sep)` - `slice(list, start, end)`. - Collections: - `len(list)`, - `find(haystack, needle)` returns index or -1. - Ternary expression: `when(condition, valueIfTrue, valueIfFalse)`. ### Conditionals Boolean expressions: - Operators: `=`, `!=`, `>`, `<`, `>=`, `<=`, logical `&&`, `||`, and negation `!`. - Equality uses single `=` inside expressions (context-specific to Cottle) rather than `==`. If statement: ```cottle {{if condition:VALUE}} ``` Compound conditionals: ```cottle {{if ((condition1 && condition2) || condition3):VALUE}} ``` Multiple conditions: ```cottle {{if condition1:VALUE1^ elif condition2:VALUE2^ else:VALUE3}} ``` After the colon `:`, all whitespace is treated as literal until the next line ending character `^`. You can also nest template blocks: ```cottle {{if condition1:{{if condition2:VALUE}}}} ```