[ci] do not fail on Docker build failure
[ci] do not fail on Docker build failure

Since the docker image is only experimentally supported and causing
build errors, we only attempt to build it but don't fail when it fails.

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan