Allow overriding the containerName

[felfert]

This PR adds a new optional parameter cachedImageTag which allows overriding the otherwise
automatically generated image tag. Reading #55 #100 and #56, I came to the conclusion, that having
the freedom to completely override the image tag would be most useful. There are also 2 other usecases for this:

• The default naming is not very userfriendly. After all, the image tags are published as "packages" in the repository of the invoker. Therefore it is desirable to have a self-explatatory name. For example in this project, I create a rust toolchain with one additional build dependency. This is useful for other projects too, so I want the name to reflect this.
• Sometimes one DOES want to share an image between different workflows or run names which is currently not possible.

One additional change was made:
The slug() function does not strictly adhere to the spec. See comments in src/run-on-arch.js, for an explanation, why I deliberately still deviate from that (but now it is still closer to the spec).

Would be appreciated, if this could be considered for inclusion in #98

Cheers
-Fritz