fix: use Docker not Dockerfile in templates

lib/config/templates/docker-digest/pr-title.hbs

-Update Dockerfile {{{depName}}} image {{{currentTag}}} digest ({{{newDigestShort}}})
+Update {{{depName}}} Docker image {{{currentTag}}} digest ({{{newDigestShort}}})

lib/config/templates/docker-pin/pr-title.hbs

-Pin Dockerfile {{{depName}}}:{{{currentTag}}} image digest
+Pin Docker {{{depName}}}:{{{currentTag}}} image digest

lib/config/templates/docker/pr-title.hbs

-Update {{{depName}}} Dockerfile tag to {{#if isMajor}}v{{{newVersionMajor}}}{{else}}v{{{newTag}}}{{/if}}
+Update {{{depName}}} Docker tag to {{#if isMajor}}v{{{newVersionMajor}}}{{else}}v{{{newTag}}}{{/if}}