Fix dockerfile to handle updates to conda clean

[BenGalewsky]

Problem

Updates to conda have broken the Dockerfile.

usage: conda clean [-h] [-a] [-i] [-p] [-t] [-f]
#7 553.5                    [-c [TEMPFILES [TEMPFILES ...]]] [-l] [-d] [--json] [-q]
#7 553.5                    [-v] [-y]
#7 553.5 conda clean: error: argument -p/--packages: ignored explicit argument 'sy'
------
executor failed running [/bin/sh -c conda install --yes     -c conda-forge     lz4     xrootd==5.1.1     tini==0.18.0     && conda clean -tipsy]: exit code: 2

Approach

Change conda clean -tipsy to the new form of conda build purge-all && conda clean -ti

This required installing the new tool, conda build