Fix docker build after buildkit change

The CI has started reporting

17 |
18 | WORKDIR /build/.git
19 | >>> COPY ./.git .
20 |
21 | WORKDIR /build/src

e.g. https://github.com/EventStore/EventStore/actions/runs/8295795061/job/22734132242

It may be that this commit moby/buildkit@a8d926a is causing the COPY line to be detected as a git ref now (because it ends in .git) but it isn't

Adding the slash prevents this and doesn't change the behaviour of the COPY line (the commit SHA is still printed correctly in the ES VERSION line)

Dockerfile

@@ -16,7 +16,7 @@ RUN dotnet restore --runtime=${RUNTIME}
 COPY ./src .
 
 WORKDIR /build/.git
-COPY ./.git .
+COPY ./.git/ .
 
 WORKDIR /build/src
 RUN find /build/src -maxdepth 1 -type d -name "*.Tests" -print0 | xargs -I{} -0 -n1 sh -c \