Fix vcpkg git fetch in Docker build images

The Docker build images were failing to fetch the vcpkg tag because
'git fetch origin ' doesn't work for git tags. Changed to
'git fetch origin tag ' to properly fetch the annotated tag
from the remote repository.

Author: jrosskopf

.github/docker/linux_amd64/Dockerfile

@@ -59,7 +59,7 @@ RUN mkdir /vcpkg && \
     cd /vcpkg && \
     git init && \
     git remote add origin $vcpkg_url && \
-    git fetch origin $vcpkg_tag && \
+    git fetch origin tag $vcpkg_tag && \
     git checkout $vcpkg_tag && \
     ./bootstrap-vcpkg.sh
 ENV VCPKG_ROOT=/vcpkg

.github/docker/linux_arm64/Dockerfile

@@ -62,7 +62,7 @@ RUN mkdir /vcpkg && \
     cd /vcpkg && \
     git init && \
     git remote add origin $vcpkg_url && \
-    git fetch origin $vcpkg_tag && \
+    git fetch origin tag $vcpkg_tag && \
     git checkout $vcpkg_tag && \
     ./bootstrap-vcpkg.sh
 ENV VCPKG_ROOT=/vcpkg