ci: Update bionic docker images to save upgrade time in builds.
ci: Update bionic docker images to save upgrade time in builds.

eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,synicalsyntax/zulip,rht/zulip,timabbott/zulip,synicalsyntax/zulip,eeshangarg/zulip,shubhamdhama/zulip,zulip/zulip,synicalsyntax/zulip,punchagan/zulip,punchagan/zulip,timabbott/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,punchagan/zulip,brainwane/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,synicalsyntax/zulip,brainwane/zulip,shubhamdhama/zulip,andersk/zulip,showell/zulip,timabbott/zulip,kou/zulip,rht/zulip,showell/zulip,shubhamdhama/zulip,synicalsyntax/zulip,kou/zulip,brainwane/zulip,andersk/zulip,kou/zulip,showell/zulip,kou/zulip,showell/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,punchagan/zulip,brainwane/zulip,brainwane/zulip,hackerkid/zulip,rht/zulip,timabbott/zulip,synicalsyntax/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,timabbott/zulip,zulip/zulip,punchagan/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,synicalsyntax/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,timabbott/zulip,eeshangarg/zulip,shubhamdhama/zulip,kou/zulip,rht/zulip,andersk/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,hackerkid/zulip