Dockerfile.dapper: set $HOME properly

`$HOME` refers to `$DAPPER_SOURCE`, which is set in the same expression
and is thus not visible at the time of substitution.

This problem is not immediately visible with Docker, Inc.'s docker
merely because it resets an unset `$HOME` to `/root` (but still breaking
the Go cache). Under podman, this problem is immediately visible because
an unset `$HOME` remains unset and subsequently breaks the `go generate`
invocation.

Fixes #9089.

Signed-off-by: Ivan Shapovalov <intelfx@intelfx.name>

Dockerfile.dapper

@@ -56,9 +56,10 @@ ENV DAPPER_RUN_ARGS="--privileged -v k3s-cache:/go/src/github.com/k3s-io/k3s/.ca
     DAPPER_SOURCE="/go/src/github.com/k3s-io/k3s/" \
     DAPPER_OUTPUT="./bin ./dist ./build/out ./build/static ./pkg/static ./pkg/deploy" \
     DAPPER_DOCKER_SOCKET=true \
-    HOME=${DAPPER_SOURCE} \
     CROSS=true \
     STATIC_BUILD=true
+# Set $HOME separately because it refers to $DAPPER_SOURCE, set above
+ENV HOME=${DAPPER_SOURCE}
 
 WORKDIR ${DAPPER_SOURCE}