Un-deprecate auto-creation of host directories for mounts

[thaJeztah]

Auto-creation of host-directories was marked deprecated in
Docker 1.9, but was decided to be too much of an backward-incompatible
change, so it was decided to keep the feature.

fixes #21652, refs #19953 (comment))

[cpuguy83]

LGTM

[calavera]

🎉 LGTM

[glensc]

Can this be addressed with some new mount option?

docker run --rm -it -v ~/.my.cnf:/root/.my.cnf:no-create busybox

[thaJeztah]

@glensc you can now use the --mount flag instead;

docker run --mount type=bind,source=~/.my.cnf,target=/root/.my.cnf busybox

The --mount type=bind..... flag will produce an error if the source file/directory does not exist

[glensc]

@thaJeztah awesomeness!

this is certainly good, because paths that are expected to be files became dirs create weird application failures :)

is there a solution for soft fail too? i.e don't create bind if source missing? as I want to add such bind to docker compose and not all users (who use the compose) may have the file present.

[thaJeztah]

is there a solution for soft fail too? i.e don't create bind if source missing? as I want to add such bind to docker compose and not all users (who use the compose) may have the file present.

Not directly; in general I would recommend bind-mounting directories instead of files. Mounting files can have odd behavior

For example, binds in Linux are based on the file/directory's inode, not their path. Some editors do not edit a file in place, but instead: save to a temporary file, then move the original file, and write the updated file to the old location. In that situation, the container would still be using the old file (before editing), which can be unexpected.

So, instead of bind-mounting ~/.my.cnf, bind-mount (for example) ~/configuration-files/ (and for that case you could use the old "auto-create" feature); if the user doesn't have that directory, docker creates an empty directory, but if it exists, it will be mounted.