compare numbers in tuning init script

[mxfactorial]

current

the if [ ${TS_TUNE_MEMORY} -gt ${FREE_BYTES} ] comparison in 772345 was not likely reached before adding cgroups v2 in https://github.com/timescale/timescaledb-docker/pull/262

expected

test for numeric TS_TUNE_MEMORY before referencing it in an integer comparison

*double brackets used similarly to postgres base image