Pin docker base image

This reduces the consequence of a base image for this action being
tampered with, since action consumers can pin the action sha and
be a bit more sure what code is running.

Signed-off-by: Nikolai R Kristiansen <nikolai.kristiansen@remarkable.no>

Author: nikolaik

Dockerfile

@@ -1,6 +1,6 @@
-FROM webispy/checkpatch
+FROM webispy/checkpatch@sha256:e84b5049be1410395ecca4547e14b1d209fb9f3327383bef83b25aeb666a8304
 
 COPY entrypoint.sh /entrypoint.sh
 COPY review.sh /review.sh
 
-ENTRYPOINT ["/entrypoint.sh"]
+ENTRYPOINT ["/entrypoint.sh"]