Add "--file" option to docker build

[thedeeno]

This is an enhancement to the core/api.

It should be possible to specify the dockerfile for builds instead of forcing it to be ./Dockerfile. This gives users more flexibility in their build process (allowing multiple dockerfiles in the same repo) while also maintaining security (cwd is still the context for the build).

It also gives users the freedom to name build files with other conventions, for instance if they prefer using extensions (e.g. foo.docker)

[tianon]

This is a duplicate of #2112.

[thedeeno]

Planning on a pull request and wanted my own issue number for my branch.