Interpreting and analysing a poem is not necessarily a matter of finding the right answer. Poems are complex creations and are open to many different interpretations.
Your interpretation is as valid as anyone else's - as long as you can back it up with suitable evidence from the text.
Docker could be interpreted as exploring any - or all - of the following themes.
The man described in the poem is very much identified here by his job, as shown by the title of the poem.
The shipbuilding imagery used in the poem even associates the man’s job with his features and characteristics.
For example, the simile describing how his “cap juts like a gantry’s crossbeam” creates an image of a hard, chiselled face.
In a shockingly matter-of-fact way, the speaker observes “That fist would drop a hammer on a Catholic”.
The violent hatred the docker has for Catholics is reinforced by the following line – “Oh yes, that sort of thing could start again”.
This is almost a warning from the speaker about what men like this could do.
The subject of the poem is portrayed as the silent and tough masculine stereotype, common in 1960s Northern Ireland.
In the final stanza he is described as “Clearly used to silence”.
The observation that his “wife and children will be quiet/ At slammed door” suggests that even in his own home he intimidates his family.