Docs: Add example exclude entry matching all tags

[dgoodlad]

There was no example of the match pattern used other than "patterns (in the grep sense)", which tripped me up as I was using unescaped + in my patterns which didn't match anything. An example here should help avoid others' confusion 😄

[mattnworb]

Thanks @dgoodlad !

Small question though - the script basically passes the output of docker images (after some transforms) to the standard input of grep -f <file containing excludes>. So instead of matching redis:[^ ]\+ (to match any tag of the redis image I presume) couldn't you also just have the pattern be redis: ?

[dgoodlad]

@mattnworb I tried that, but discovered after winding through the code that the patterns here get transformed by

docker-gc/docker-gc

Line 113 in 0c80872

sed 's/^\(.*\)$/ \1 /' $EXCLUDE_FROM_GC | sed '/^ *$/d' > $PROCESSED_EXCLUDES

, adding a space to the beginning and end, like so:

"redis:" -> " redis: "
"redis:[^ ]\+" -> " redis:[^ ]\+ "

The former will not match redis:latest, while the latter will.

The extra spaces were what made figuring out exactly how to use the "grep style" pattern matching work difficult 😄

[mattnworb]

Gotcha, thanks for clarifying.