builder/docker: improve errors
builder/docker: improve errors

tsuru/tsuru,tsuru/tsuru,ggarnier/tsuru,ArxdSilva/tsuru,ArxdSilva/tsuru,tsuru/tsuru,ggarnier/tsuru,cezarsa/tsuru,cezarsa/tsuru,ggarnier/tsuru,guilhermebr/tsuru,cezarsa/tsuru,ggarnier/tsuru,cezarsa/tsuru,guilhermebr/tsuru,ArxdSilva/tsuru,tsuru/tsuru,guilhermebr/tsuru,guilhermebr/tsuru,ArxdSilva/tsuru