Bump Dockerfile Go version
Bump Dockerfile Go version

ericgerling/alertmanager,GoogleCloudPlatform/alertmanager,sarathsp06/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,tinytub/alertmanager,brancz/alertmanager,sarathsp06/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,joehandzik/alertmanager,joehandzik/alertmanager,Aloomaio/alertmanager,tbouvet/alertmanager,sarathsp06/alertmanager,novilabs/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,Kellel/alertmanager,prometheus/alertmanager,tinytub/alertmanager,Aloomaio/alertmanager,ericgerling/alertmanager,brancz/alertmanager,brancz/alertmanager,tbouvet/alertmanager,tinytub/alertmanager,novilabs/alertmanager,brunoqc/alertmanager,brancz/alertmanager,tyrken/alertmanager,brunoqc/alertmanager,Kellel/alertmanager,novilabs/alertmanager,tbouvet/alertmanager,novilabs/alertmanager,brunoqc/alertmanager,Aloomaio/alertmanager,tyrken/alertmanager,Kellel/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,GoogleCloudPlatform/alertmanager,ericgerling/alertmanager