docs: clarify leading and trailing slashes in add/copy source path

[dvdksn]

Leading and trailing slashes in the source path for ADD and COPY, when
adding/copying files from the build context, have no effect.

• relates to Explicitly state whether leading and trailing slashes hold any meaning for the source of ADD/COPY commands docker/docs#21825

[thaJeztah]

Do we document somewhere that for the destination the path may be resolved relative to the WORKDIR that's set, so if it's a relative path (I think) we resolve it relative to the active WORKDIR

[dvdksn]

Doesn't look like we describe that too well... only indirectly. I guess we could do with a bit more clarity there too. I can do a followup.

[dvdksn]

Oh, never mind me. We do describe it. I'm just bad at reading documentation.

[cowwoc]

In the example you gave, does COPY ../something /something get treated as COPY something /something? It's not immediately clear that this is the case...

[dvdksn]

Yes, COPY ../something /something and COPY something /something are equivalent

[cowwoc]

@dvdksn Any chance you could add this clarification to the updated document? Here is what the updated version would look like:

Specifying a source path with a leading slash or one that navigates outside the
build context, such as COPY ../something /something, automatically removes
any parent directory navigation (../), making COPY ../something/ /something equivalent to COPY something /something.