x509 Go package - ParsePKIXPublicKey is to DER or PEM?

The issue here is understanding the difference between x509.ParsePKCS1PublicKey (PKCS#1) and x509.ParsePKIXPublicKey (PKCS#8).

Usually when the PEM header has the type as RSA PUBLIC KEY, it is referring to a PKCS#1 encoded RSA public key, which is defined in RFC 8017 (PKCS#1) as:

RSAPublicKey ::= SEQUENCE {
     modulus           INTEGER,  -- n
     publicExponent    INTEGER   -- e
 }

You haven't actually provided the body of your public key (it would be safe to do so), but it is a fair assumption that, if decoding the key using x509.ParsePKIXPublicKey failed, your key is likely in the above format (x509.ParsePKIXPublicKey uses PKCS#8 encoding).

If this is the case, you should be able to get an rsa.PublicKey from the file using the following code (don't forget to add the error handling):

rawPem, _ := ioutil.ReadFile("./public.key")
pemBlock, _ := pem.Decode(rawPem)
publicKey, _ := x509.ParsePKCS1PublicKey(pemBlock.Bytes)

If this doesn't solve your problem, try pasting the key you have into this site to see what ASN.1 structure it uses. For reference, the key I used to test this is included here:

-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEApW1W9dnfdFF7FHrq6HPveR/9T+nM70yO7QOGytR0j/chMBJcJBjG
hJOuKPFbkVyS+BE/4M8CojLgvz4ex82Re0sFa5TqnoWvuP5P4vktR6M5W53sTW3y
gUnfF/oHcEmARQ1xKZdgVnlIfrdbpjecPyLi1Ng4HmhEfCFUOW64koxpb4XeH5O5
q+vc/731ExVOYBU8Sl6kPdjpJuVjS3DHKAVgfVEhscXd3JDjDuMDT3w1IYNb5c2s
wHE55q4Jnc1cr42jdynnkXzmuOGo2C6yD95kbBDLp7wSiBxaMA8gbRkzWJ99T+6l
KsKG2zfndMF3jZW1v1wWiEbYRN07qbN0NQIDAQAB
-----END RSA PUBLIC KEY-----