documentation for variable expansion

Signed-off-by: Tristan Stenner <[email protected]>

frontend/dockerfile/docs/reference.md

@@ -287,17 +287,30 @@ modifiers as specified below:
   will be that value. If `variable` is not set then `word` will be the result.
 - `${variable:+word}` indicates that if `variable` is set then `word` will be
   the result, otherwise the result is the empty string.
+- `${variable#pattern}` will return the value of `variable` with the shortest
+  expansion of the pattern stripped from the beginning
+- `${variable##pattern}` will return the value of `varaible` with the *longest*
+  expansion of the pattern stripped from the beginning,
+  e.g. for `sheep=baaah` the expression `${sheep#b*a}` will yield `h` rather
+  than `aah`.
+- `${variable%pattern}` and `${variable%%pattern}` will strip from the end,
+  respectively.
 
 In all cases, `word` can be any string, including additional environment
 variables.
 
+`pattern` can be a glob pattern where `?` will match any single character
+and `*` any number of characters (including zero).
+
 Escaping is possible by adding a `\` before the variable: `\$foo` or `\${foo}`,
 for example, will translate to `$foo` and `${foo}` literals respectively.
 
 Example (parsed representation is displayed after the `#`):
 
 ```dockerfile
-FROM busybox
+ARG IMAGE=busybox:latest
+# FROM busybox:stable
+FROM ${IMAGE%:*}:stable
 ENV FOO=/bar
 WORKDIR ${FOO}   # WORKDIR /bar
 ADD . $FOO       # ADD . /bar