includes a new config option `dockerBuildFlags` that allow injecting
arbitrary flags to the build command. Internally, the `arg` function on
the docker image class which is used to accumulate the `build-arg`
options has been converted to make use of the generic flag method.

Flags are normalized in this fashion.

* If the key does not start with a `-` it will be prepended
* all occurences of `_` will be re-written as `-`
* Single letter keys are considered shorthands e.g. `p` becomes `-p`
* Multi letter keys are considered long form e.g. `foo_bar` becomes `--foo-bar`
* If the value is an array, the flag is repeated for each occurance
* A `null` value may be used to omit the value and only inject the flag itself

includes support for the build flags cache-from and cache-to

fixes: #35