Improve dockerfile for restore speed

[cdhanna]

COPY services/Serv/Serv.csproj ${BEAM_CSPROJ_PATH}
COPY services/Dato/Dato.csproj /subsrc/services/Dato/Dato.csproj
RUN dotnet restore ${BEAM_CSPROJ_PATH} -p:BeamableVersion=${BEAM_VERSION} -a $TARGETARCH

If you do the dotnet restore as a special pre-step, and then add the --no-restore flag to the publish command, docker can layer-cache the restore, and it only breaks when you're csproj file changes (like adding a nuget reference).