docker: Explicitly set nginx types_hash_bucket_size

As discovered by the inimitable bug-hunter @nigelbabu, on the version of
Nginx in the image, the value of types_hash_bucket_size is set at
runtime to the processor line cache size.

This means that on hardware with small cache lines (16 bytes) the image
can fail to start with:

    nginx: [emerg] could not build the types_hash, you should increase
    either types_hash_max_size: 1024 or types_hash_bucket_size: 32

This commit sets types_hash_bucket_size and types_hash_max_size to the
values used by latest Nginx (1.7.2 at the time of writing).