By default, there's an expectation that the app's build.boot
file defines build task that takes care of all sorts of build-time
concerns (e.g. pulling dependencies from repos eagerly). If a more
complicated logic is desired at build-time, it can be overriden by
specifying BOOTBUILD_CMD var that contains, say, a more complex boot
pipeline. For instance:
BOOTBUILD_CMD="boot foo -x -- bar --blah -q -- qaz"
To expose more config vars at build-time, set a
BOOTBUILD_CONFIG_WHITELIST config var containing a space-delimited
list of config var names. Note that this can result in unpredictable
behaviour since changing your app's config does not result in a
rebuild of your app. So it's easy to get into a situation where your
build is broken, but you don't notice it until later when you
push. For this reason it's recommended to take care with this feature
and always push after changing a whitelisted config value.
Thanks to the authors of various official Heroku buildpacks for providing excellent documentation and clearly written source code. In particular, config var whitelisting has been inspired by Clojure buildpack.