Add support for more advanced ${xxx:...} syntax
Just ${xxx:+...} and ${xxx:-...} for now
@@ -113,15 +113,27 @@ images.
> replacement at the time. After 1.3 this behavior will be preserved and
> canonical.
-Environment variables (declared with [the `ENV` statement](#env)) can also be used in
-certain instructions as variables to be interpreted by the `Dockerfile`. Escapes
-are also handled for including variable-like syntax into a statement literally.
+Environment variables (declared with [the `ENV` statement](#env)) can also be
+used in certain instructions as variables to be interpreted by the
+`Dockerfile`. Escapes are also handled for including variable-like syntax
+into a statement literally.
Environment variables are notated in the `Dockerfile` either with
`$variable_name` or `${variable_name}`. They are treated equivalently and the
brace syntax is typically used to address issues with variable names with no
whitespace, like `${foo}_bar`.
+The `${variable_name}` syntax also supports a few of the standard `bash`
+modifiers as specified below:
+
+* `${variable:-word}` indicates that if `variable` is set then the result
+ will be that value. If `variable` is not set then `word` will be the result.
+* `${variable:+word}` indiates that if `variable` is set then `word` will be
+ the result, otherwise the result is the empty string.
+
+In all cases, `word` can be any string, including additional environment
+variables.
+
Escaping is possible by adding a `\` before the variable: `\$foo` or `\${foo}`,
for example, will translate to `$foo` and `${foo}` literals respectively.
