Create ENM base images

[brownjohnf]

We should build + push the Dockerfile in here, so that instructions to users can be to just use FROM resin/enm:v1.0.3 (or whatever tag they want) in their gateway applications.

[jbaldwinroberts]

I'm not sure if this is such a good idea, unless we make one for each arch and language - otherwise the user will do FROM resin/enm:v1.0.3 and then have to install the whole node toolchain or whichever language they want to use

[jbaldwinroberts]

I guess we could do this with multistage builds, something like:

FROM resin/%%RESIN_MACHINE_NAME%%-enm:v6.0.0 as enm
FROM resin/%%RESIN_MACHINE_NAME%%-node:latest
COPY --from=enm ./edge-node-manager ./edge-node-manager
....

This means we only have to create a base enm image for each arch