Feature: Add BUILD_DEBUG argument when building Docker image

[oobayly]

When creating #750 I needed to be enable debugging for the docker image. Passing an optional BUILD_DEBUG argument with a non-empty value:

• Runs cmake with the -DCMAKE_BUILD_TYPE=Debug flag
• Doesn't strip the executables after they're build
• Installs gdb

docker build --build-arg BUILD_DEBUG=1 --tag tilemaker/debug .