ignore .docker-tag files created on the build process

These files are not relevant so git-ignore them, leading to a less
noisy git-status.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I1342d0008ceeeb5f581a1accc04c489da330b53e
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..06bcd32
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.docker-tag
\ No newline at end of file