Fix docker not working in real action

Dockerfile

@@ -5,4 +5,6 @@ LABEL com.github.actions.description="Check your code for `FIXME` labels"
 LABEL com.github.actions.icon="code"
 LABEL com.github.actions.color="yellow"
 
-ENTRYPOINT ["lib/entrypoint.sh"]
+COPY lib/entrypoint.sh /entrypoint.sh
+COPY lib/git-grep-problem-matcher.json /git-grep-problem-matcher.json
+ENTRYPOINT ["/entrypoint.sh"]

lib/entrypoint.sh

@@ -2,7 +2,13 @@
 
 set -e
 
-echo "::add-matcher::lib/git-grep-problem-matcher.json"
+# With problem matchers in a container, the matcher config MUST be available
+# outside the container on the VM so we will just copy it into the workspace.
+# See: https://github.com/actions/toolkit/issues/205#issuecomment-557647948
+matcher_path=`pwd`/git-grep-problem-matcher.json
+cp /git-grep-problem-matcher.json "$matcher_path"
+
+echo "::add-matcher::git-grep-problem-matcher.json"
 
 tag="FIXME"
 result=$(git grep --no-color -n -e "${tag}:")