hostinfo,build_docker.sh,tailcfg: more reliably detect being in a container (#13826)

Our existing container-detection tricks did not work on Kubernetes,
where Docker is no longer used as a container runtime. Extends the
existing go build tags for containers to the other container packages
and uses that to reliably detect builds that were created by Tailscale
for use in a container. Unfortunately this doesn't necessarily improve
detection for users' custom builds, but that's a separate issue.

Updates #13825

Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

build_docker.sh

@@ -56,6 +56,7 @@ case "$TARGET" in
         -X tailscale.com/version.gitCommitStamp=${VERSION_GIT_HASH}" \
       --base="${BASE}" \
       --tags="${TAGS}" \
+      --gotags="ts_kube,ts_package_container" \
       --repos="${REPOS}" \
       --push="${PUSH}" \
       --target="${PLATFORM}" \
@@ -72,6 +73,7 @@ case "$TARGET" in
         -X tailscale.com/version.gitCommitStamp=${VERSION_GIT_HASH}" \
       --base="${BASE}" \
       --tags="${TAGS}" \
+      --gotags="ts_kube,ts_package_container" \
       --repos="${REPOS}" \
       --push="${PUSH}" \
       --target="${PLATFORM}" \