Fix dockerfile-ts-mode line continuation indentation (bug#61131)

Fix dockerfile-ts-mode line continuation indentation (bug#61131)

Without this rule, line continuations are only indented after entering
the contents for the line and hitting TAB or RET.

For example:
```
EXPOSE 1 \
```

After hitting RET to go to the next line, point would end up at BOL
instead of lining up with the previous entry, like so:
```
EXPOSE 1 \
2
```

The new rule will indent it as so:
```
EXPOSE 1 \
       2
```

* lisp/progmodes/dockerfile-ts-mode.el:
(dockerfile-ts-mode--indent-rules): New rule.
(dockerfile-ts-mode--line-continuation-p)
(dockerfile-ts-mode--line-continuation-anchor): New functions.