Single Quotes for SPC

[DirectXMan12]

When setting up the SPC example, I had to use single quotes for the INSTALL label, otherwise docker would try to do environment variable substitution (and come up with empty variables for ${DATADIR}, etc) during docker build.

[rhatdan]

DATADIR should have been set by the atomic command?

[nalind]

It would be, but the docker image builder interpolates environment variables in LABEL statements, so instead of getting "-e DATADIR=${DATADIR}" in the image metadata, "atomic" only gets "-e DATADIR=".