Fix Docker build

[DRSchlaubi]

Currently the docker build creates an incompatible binary for alpine, when compiling on non musl operating systems causing a runtime error when starting the container

[jessepeterson]

How about something less opinionated:

diff --git a/README.md b/README.md
index 0f765b0..aea5980 100644
--- a/README.md
+++ b/README.md
@@ -163,7 +163,7 @@ If you're not sure which SHA-256 hash (for a specific CA) to use, you can use th
 
 ```sh
 # first compile the Docker binaries
-make docker
+CGO_ENABLED=0 make docker
 
 # build the image
 docker build -t micromdm/scep:latest .