Restore support for alt dockerfiles (removing hardcoded "Dockerfile")

[PlugNPush]

Resolves #977
Resolves #929
This commit restores setting a build directory as a string and looking for all available dockerfile alt names.
I strongly believe this is a bugfix, not modifying a compatibilty feature, as this was working that way in the older version 1.0.6.

The code for checking all the dockerfile alt names is already there, but "Dockerfile" was hardcoded in the normalize function, instead of being set to None to allow further alt dockerfile names search later in build_one().

build_one() already supports dockerfile being set to None (and that is visibly supposed to be that way).

With this PR, having a podman-compose.yaml with:
build: .
No longer throws "No Dockerfile found in CTX" when using a Containerfile (or other alt name).

[p12tic]

Thanks a lot for the PR. It looks good from the first sight, didn't look in depth yet.

We will need tests that cover all 4 cases:

• build: .
• build dictionary but not dockerfile
• build dictionary with dockerfile: Dockerfile
• build dictionary with dockerfile: Dockerfile.custom

Therefore it's not appropriate just to change existing tests, we will need additional cases. Hopefully this is mostly copy-paste and can be done without much effort.

Thanks again!

[PlugNPush]

Thanks a lot for the PR. It looks good from the first sight, didn't look in depth yet.

We will need tests that cover all 4 cases:

* `build: .`

* build dictionary but not dockerfile

* build dictionary with `dockerfile: Dockerfile`

* build dictionary with `dockerfile: Dockerfile.custom`

Therefore it's not appropriate just to change existing tests, we will need additional cases. Hopefully this is mostly copy-paste and can be done without much effort.

Thanks again!

All these cases are already included in test_normalize_final_build.py, have a look at the changes :)
They used to expect "dockerfile": "Dockerfile" in every test case, which is incorrect when trying to support alt dockerfile names.

[p12tic]

All these cases are already included in test_normalize_final_build.py, have a look at the changes :)
They used to expect "dockerfile": "Dockerfile" in every test case, which is incorrect when trying to support alt dockerfile names.

You're right. Turns out that the caveat of didn't look in depth yet was worth writing.

[p12tic]

Looks great, thanks!

Could you please add a newsfragment with a description of what this fixes? This is just one line file in newsfragments directory, see newsfragments/README.md for an explanation and latest release on how other release notes look like.

Also, could you combine the commits into one? Whole PR is semantically one change with tests, so there's no need to split it across multiple commits to show how the code evolved until it arrived to correct conclusion.

[PlugNPush]

All done! @p12tic

[p12tic]

Thanks!