provision/docker: use safe.Writer and safe.Reader in shell
provision/docker: use safe.Writer and safe.Reader in shell

Fixing a data race with the streams.

RichardKnop/tsuru,gciotta/tsuru,tuxmonteiro/tsuru,hcxiong/tsuru,gciotta/tsuru,firebitsbr/tsuru,ArxdSilva/tsuru,jblancett/tsuru,tuxmonteiro/tsuru,4eek/tsuru,cgvarela/tsuru,kgrvamsi/tsuru,hugoantunes/tsuru,tsuru/tsuru,cezarsa/tsuru,mderaldo/tsuru,firebitsbr/tsuru,hcxiong/tsuru,ggarnier/tsuru,Endika/tsuru,ggarnier/tsuru,cezarsa/tsuru,tsuru/tsuru,mderaldo/tsuru,Endika/tsuru,tsuru/tsuru,cezarsa/tsuru,gwmoura/tsuru,firebitsbr/tsuru,gciotta/tsuru,guilhermebr/tsuru,ArxdSilva/tsuru,4eek/tsuru,cgvarela/tsuru,jblancett/tsuru,4eek/tsuru,guilhermebr/tsuru,ArxdSilva/tsuru,hcxiong/tsuru,tsuru/tsuru,gwmoura/tsuru,ggarnier/tsuru,tuxmonteiro/tsuru,mderaldo/tsuru,ggarnier/tsuru,cezarsa/tsuru,RichardKnop/tsuru,ArxdSilva/tsuru,jblancett/tsuru,guilhermebr/tsuru,kgrvamsi/tsuru,kgrvamsi/tsuru,cgvarela/tsuru,Endika/tsuru,RichardKnop/tsuru,guilhermebr/tsuru,hugoantunes/tsuru,gwmoura/tsuru