build.bash: workaround a bug in docker buildx build CLI

[Al2Klimov]

--build-context dirs seem to have to end with a / to be accepted.

This blocks:

• Process config::update/delete cluster events gracefully icinga2#9980
• Document how to enable/disable Debug Output on the fly icinga2#9981
• ApiListener#AddConnection(): start connect timeout after DNS resolution icinga2#9982

[julianbrost]

I already ran into this myself locally. There seems to be some change in buildx 0.12.0 that results in this being interpreted as a git URL now, there's no such change mentioned in the changelog though.