Add embed.js to Dockerfile

[micw]

Description

Adds embed.js to the docker image

Motivation and Context

Deliver embed.js along with meshviewer, so that embedding pages can refer to the version that blongs to this meshviewer version.

How Has This Been Tested?

Only reviewed (just adds 1 file to the docker image)

Screenshots/links:

Checklist:

• My code follows the code style of this project. (CI will test it anyway and also needs approval)
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

[github-actions]

Preview deployed: https://freifunk.github.io/meshviewer/pr-preview/pr-405/

Updated for commit 0a02491.

[Jeidnx]

I'm not quite sure about this. Do i understand correctly that you want to load <meshviewer_url>/embed.js in your parent page? As i understand it the embed.js is used to synchronize the URL of the parent with the child, which makes sense during development / debugging. What would be the use case for embedding meshviewer but still synchronizing the parent URL?

I'm not against merging this, i would just like to understand / get more context before approving.

[micw]

We have https://karte.freifunk-leipzig.de/#/de/map (meshviewer) and https://freifunk-leipzig.de/map/ (Website with meshviewer in iframe). The embed feature (allready merged) allows to deep-link from the website into the map and also update the map url hashes on the website.
After switching from my (old) fork to your official mapviewer, I noticed that our website does not deliver the "embed.js" script but directly gets it from the mapviewer server (worked on my old for, but not on the official since the file is not bundled in the docker image).

I thought about the options:

1. deliver the file along with the website
2. deliver the file along with meshviewer

While 1st would be the easy/fast way, it has several drawbacks:
a) I would copy a meshviewer atrefact (the embed.js from the embedding example) to our website
b) if the file is updated to be compatible with newer meshviewer versions, we (and others) would have to update the website too

[skorpy2009]

LGTM