Document the `ExecStart=` behavior for the dockerd config

pkg/provision/buildroot.go

@@ -73,6 +73,13 @@ Environment=DOCKER_RAMDISK=yes
 {{range .EngineOptions.Env}}Environment={{.}}
 {{end}}
 
+# This file is a systemd drop-in unit that inherits from the base dockerd configuration.
+# The base configuration already specifies an 'ExecStart=...' command. The first directive
+# here is to clear out that command inherited from the base configuration. Without this,
+# the command from the base configuration and the command specified here are treated as
+# a sequence of commands, which is not the desired behavior, nor is it valid -- systemd
+# will catch this invalid input and refuse to start the service with an error like:
+#  Service has more than one ExecStart= setting, which is only allowed for Type=oneshot services.
 ExecStart=
 ExecStart=/usr/bin/docker daemon -H tcp://0.0.0.0:{{.DockerPort}} -H unix:///var/run/docker.sock --tlsverify --tlscacert {{.AuthOptions.CaCertRemotePath}} --tlscert {{.AuthOptions.ServerCertRemotePath}} --tlskey {{.AuthOptions.ServerKeyRemotePath}} {{ range .EngineOptions.Labels }}--label {{.}} {{ end }}{{ range .EngineOptions.InsecureRegistry }}--insecure-registry {{.}} {{ end }}{{ range .EngineOptions.RegistryMirror }}--registry-mirror {{.}} {{ end }}{{ range .EngineOptions.ArbitraryFlags }}--{{.}} {{ end }}
 ExecReload=/bin/kill -s HUP $MAINPID