Feature/751 Add optional BUILD_DEBUG Docker build argument

[oobayly]

See #751

Makes it easier to 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 .

[systemed]

Thank you!