.dockerignore is included in build context hash

[ethanwu10]

.dockerignore is included in the list of files to hash when hashing the build context, which can lead to unnecessary container rebuilds. Since the only effects of a .dockerignore are to modify what files are included in the build context, which are already captured via evaluation of the rules in the file, the contents of the file add no information (except potentially extraneous information in the form of formatting changes), and thus should not belong in the hash.